In case "Cookie" header is sent by client, multiple cookie pairs were
incorrectly split by a semicolon and comma.

Now they are split by a semicolon only.

For example, next variables will be found for "Cookie: a=b, c=d; e=f":
- $cookie_a: "b, c=d"
- $cookie_e: "f"

Closes #1042 on GitHub.

This allows to process a port subcomponent and save it in r->port
in a unified way, similar to r->headers_in.server.  For HTTP/1.x
request line in the absolute form, r->host_end now includes a port
subcomponent, which is also consistent with HTTP/2 and HTTP/3.

The function contains mostly HTTP/1.x specific request processing,
which has no use in other protocols.  After the previous change in
HTTP/2, it can now be hidden.

This is an API change.

The change implements processing upstream early hints response in
ngx_http_proxy_module and ngx_http_grpc_module.  A new directive
"early_hints" enables sending early hints to the client.  By default,
sending early hints is disabled.

Example:

    map $http_sec_fetch_mode $early_hints {
        navigate $http2$http3;
    }

    early_hints $early_hints;

    proxy_pass http://example.com;

The directive allows to pass upstream response trailers to client.

Multi headers are now using linked lists instead of arrays.  Notably,
the following fields were changed: r->headers_in.cookies (renamed
to r->headers_in.cookie), r->headers_in.x_forwarded_for,
r->headers_out.cache_control, r->headers_out.link, u->headers_in.cache_control
u->headers_in.cookies (renamed to u->headers_in.set_cookie).

The r->headers_in.cookies and u->headers_in.cookies fields were renamed
to r->headers_in.cookie and u->headers_in.set_cookie to match header names.

The ngx_http_parse_multi_header_lines() and ngx_http_parse_set_cookie_lines()
functions were changed accordingly.

With this change, multi headers are now essentially equivalent to normal
headers, and following changes will further make them equivalent.

ngx_http_v2_huff_decode.c and ngx_http_v2_huff_encode.c are renamed
to ngx_http_huff_decode.c and ngx_http_huff_encode.c.