Var: Added $request_uri (as in NGINX).

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>

docs/changes.xml

@@ -55,6 +55,12 @@ variables support in the "location" option of the "return" action.
 </para>
 </change>
 
+<change type="feature">
+<para>
+added new variable $request_uri with the same meaning as in NGINX.
+</para>
+</change>
+
 <change type="feature">
 <para>
 compatibility with GCC 12.