Before Node.js v16.14.0 the "format" value in defaultResolve
was ignored so error was hidden.  For more information see:
https://github.com/nodejs/node/pull/40980

This supports a new variable $request_uri that contains the path
and the query (See RFC 3986, section 3).  Its contents are percent
encoded.  This is useful for example to redirect HTTP to HTTPS:

{
    "return": "301",
    "location": "https://$host$request_uri"
}

When <http://example.com/foo%23bar?baz> is requested, the server
redirects to <https://example.com/foo%23bar?baz>.

===

Testing:

//diff --git a/src/nxt_http_return.c b/src/nxt_http_return.c
//index 82c9156..adeb3a1 100644
//--- a/src/nxt_http_return.c
//+++ b/src/nxt_http_return.c
//@@ -196,6 +196,7 @@ nxt_http_return_send_ready(nxt_task_t *task,
    void *obj, void *data)
//         field->value = ctx->encoded.start;
//         field->value_length = ctx->encoded.length;
//     }
//+    fprintf(stderr, "ALX: target[%1$i]: <%2$.*1$s>\n",
    (int)r->target.length, r->target.start);
//
//     r->state = &nxt_http_return_send_state;
//

{
	"listeners": {
		"*:81": {
			"pass": "routes/ru"
		}
	},

	"routes": {
		"ru": [{
			"action": {
				"return": 301,
				"location": "$request_uri"
			}
		}]
	}
}

$ curl -i http://localhost:81/*foo%2Abar?baz#arg
HTTP/1.1 301 Moved Permanently
Location: /*foo%2Abar?baz
Server: Unit/1.27.0
Date: Mon, 30 May 2022 16:04:30 GMT
Content-Length: 0

$ sudo cat /usr/local/unit.log | grep ALX
ALX: target[15]: </*foo%2Abar?baz>

This supports a new option "index" that configures a custom index
file name to be served when a directory is requested.  This
initial support only allows a single fixed string.  An example:

{
	"share": "/www/data/static/$uri",
	"index": "lookatthis.htm"
}

When <example.com/foo/bar/> is requested,
</www/data/static/foo/bar/lookatthis.html> is served.

Default is "index.html".

===

nxt_conf_validator.c:

Accept "index" as a member of "share", and make sure it's a string.

===

I tried this feature in my own computer, where I tried the
following:

- Setting "index" to "lookatthis.htm", and check that the correct
  file is being served (check both a different name and a
  different extension).
- Not setting "index", and check that <index.html> is being
  served.
- Settind "index" to an array of strings, and check that the
  configuration fails:

{
	"error": "Invalid configuration.",
	"detail": "The \"index\" value must be a string, but not an array."
}

An empty string in Location was being handled specially by not sending a
Location header.  This may occur after variable resolution, so we need to
consider this scenario.

The obsolete RFC 2616 defined the Location header as consisting of an absolute
URI <https://www.rfc-editor.org/rfc/rfc2616#section-14.30>, which cannot be an
empty string.  However, the current RFC 7231 allows the Location to be a
relative URI <https://www.rfc-editor.org/rfc/rfc7231#section-7.1.2>, and a
relative URI may be an empty string <https://stackoverflow.com/a/43338457>.

Due to these considerations, this patch allows sending an empty Location header
without handling this case specially.  This behavior will probably be more
straightforward to users, too.  It also simplifies the code, which is now more
readable, fast, and conformant to the current RFC.  We're skipping an
allocation at request time in a common case such as "action": {"return": 404}

This closes #654 issue on Github.

............
Description:
............

Before this commit, the encoded URI could be calculated at
configuration time.  Now, since variables can only be resolved at
request time, we have different situations:

- "location" contains no variables:

  In this case, we still encode the URI in the conf structure, at
  configuration time, and then we just copy the resulting string
  to the ctx structure at request time.

- "location" contains variables:

  In this case, we compile the var string at configure time, then
  when we resolve it at request time, and then we encode the
  string.

In both cases, as was being done before, if the string is empty,
either before or after resolving variables, we skip the encoding.

...........
Usefulness:
...........

An example of why this feature may be useful is redirecting HTTP
to HTTPS with something like:

"action": {
    "return": 301,
    "location": "https://${host}${uri}"
}

.....
Bugs:
.....

This feature conflicts with the relevant RFCs in the following:

'$' is used for Unit variables, but '$' is a reserved character in
a URI, to be used as a sub-delimiter.  However, it's almost never
used as that, and in fact, other parts of Unit already conflict
with '$' being a reserved character for use as a sub-delimiter, so
this is at least consistent in that sense.  VBart suggested an
easy workaround if we ever need it: adding a variable '$sign'
which resolves to a literal '$'.

......
Notes:
......

An empty string is handled as if "location" wasn't specified at
all, so no Location header is sent.

This is incorrect, and the code is slightly misleading.

The Location header consists of a URI-reference[1], which might be
a relative one, which itself might consist of an empty string[2].

[1]: <https://www.rfc-editor.org/rfc/rfc7231#section-7.1.2>
[2]: <https://stackoverflow.com/a/43338457>

Now that we have variables, it's more likely that an empty
Location header will be requested, and we should handle it
correctly.

I think in a future commit we should modify the code to allow
differentiating between an unset "location" and an empty one,
which should be treated as any other "location" string.

.................
Testing (manual):
.................

{
  "listeners": {
    "*:80": {
      "pass": "routes/str"
    },
    "*:81": {
      "pass": "routes/empty"
    },
    "*:82": {
      "pass": "routes/var"
    },
    "*:83": {
      "pass": "routes/enc-str"
    },
    "*:84": {
      "pass": "routes/enc-var"
    }
  },
  "routes": {
    "str": [
      {
        "action": {
          "return": 301,
          "location": "foo"
        }
      }
    ],
    "empty": [
      {
        "action": {
          "return": 301,
          "location": ""
        }
      }
    ],
    "var": [
      {
        "action": {
          "return": 301,
          "location": "$host"
        }
      }
    ],
    "enc-str": [
      {
        "action": {
          "return": 301,
          "location": "f%23o#o"
        }
      }
    ],
    "enc-var": [
      {
        "action": {
          "return": 301,
          "location": "f%23o${host}#o"
        }
      }
    ]
  }
}

$ curl --dump-header - localhost:80
HTTP/1.1 301 Moved Permanently
Location: foo
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:06 GMT
Content-Length: 0

$ curl --dump-header - localhost:81
HTTP/1.1 301 Moved Permanently
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:08 GMT
Content-Length: 0

$ curl --dump-header - localhost:82
HTTP/1.1 301 Moved Permanently
Location: localhost
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:15 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: bar" localhost:82
HTTP/1.1 301 Moved Permanently
Location: bar
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:23 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: " localhost:82
HTTP/1.1 301 Moved Permanently
Server: Unit/1.27.0
Date: Thu, 07 Apr 2022 23:30:29 GMT
Content-Length: 0

$ curl --dump-header - localhost:83
HTTP/1.1 301 Moved Permanently
Location: f%23o#o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:22:23 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: " localhost:84
HTTP/1.1 301 Moved Permanently
Location: f%23o#o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:22:44 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: alx" localhost:84
HTTP/1.1 301 Moved Permanently
Location: f%23oalx#o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:22:52 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: a#l%23x" localhost:84
HTTP/1.1 301 Moved Permanently
Location: f%2523oa#l%2523x%23o
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:23:09 GMT
Content-Length: 0

$ curl --dump-header - -H "Host: b##ar" localhost:82
HTTP/1.1 301 Moved Permanently
Location: b#%23ar
Server: Unit/1.27.0
Date: Sat, 09 Apr 2022 11:25:01 GMT
Content-Length: 0