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.

In addition to moving memcpy() under the length condition in 15bf6d8cc,
which addressed a reported UB due to string function conventions, this
is repeated for advancing an input buffer, to make the resulting code
more clean and readable.

Additionally, although considered harmless for both string functions and
additive operators, as previously discussed in GitHub PR 866, this fixes
the main source of annoying sanitizer reports in the module.

Prodded by UndefinedBehaviorSanitizer (pointer-overflow).

The $request_port variable contains the port passed by the client in the
request line (for HTTP/1.x) or ":authority" pseudo-header (for HTTP/2 and
HTTP/3).  If the request line contains no host, or ":authority" is missing,
then $request_port is taken from the "Host" header, similar to the $host
variable.

The $is_request_port variable contains ":" if $request_port is non-empty,
and is empty otherwise.

Missed in fcf4331a0.

Previously, it misused the Host header processing resulting in
400 (Bad Request) errors for a valid request that contains both
":authority" and Host headers with the same value, treating it
after 37984f0be as if client sent more than one Host header.
Such an overly strict handling violates RFC 9113.

The fix is to process ":authority" as a distinct header, similarly
to processing an authority component in the HTTP/1.x request line.
This allows to disambiguate and compare Host and ":authority"
values after all headers were processed.

With this change, the ngx_http_process_request_header() function
can no longer be used here, certain parts were inlined similar to
the HTTP/3 module.

To provide compatibility for misconfigurations that use $http_host
to return the value of the ":authority" header, the Host header,
if missing, is now reconstructed from ":authority".

No functional changes.

Previously, when using HTTP/2 over SSL, an early hints HEADERS frame was
queued in SSL buffer, and might not be immediately flushed.  This resulted
in a delay of early hints delivery until the main response was sent.

The fix is to set the flush flag for the early hints HEADERS frame buffer.

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;