Uwsgi: ensure HTTP_HOST is set to the requested target host.

When using HTTP/2 or prior the CGI/1.1 HTTP_HOST environment variable is
set to the requested target host.

However with HTTP/3 (or with HTTP/1.1 with the host contained in the
request line) this was not happening.

The HTTP/2 module uses the ':authority' pseudo-header field to set
http_host. It was decided not to do that for HTTP/3.

Instead we create a *new* default for http_host which will now use the
HOST request header or :authority pseudo-header field.

While this is a slight change in behaviour in regards to passing HTTP
request header fields, it should be fine in practice as the HTTP host
request header and the :authority pseudo-header field should be the
same.

Also the CGI specification (RFC 3875, 4.1.18) notes

  The server SHOULD set meta-variables specific to the protocol and
  scheme for the request. Interpretation of protocol-specific
  variables depends on the protocol version in SERVER_PROTOCOL.

Link: <https://datatracker.ietf.org/doc/html/rfc3875#section-4.1.18>

src/http/modules/ngx_http_uwsgi_module.c

@@ -629,6 +629,13 @@ static ngx_str_t ngx_http_uwsgi_hide_headers[] = {
 };
 
 
+static ngx_keyval_t  ngx_http_uwsgi_headers[] = {
+    { ngx_string("HTTP_HOST"),
+      ngx_string("$host$is_request_port$request_port") },
+    { ngx_null_string, ngx_null_string }
+};
+
+
 #if (NGX_HTTP_CACHE)
 
 static ngx_keyval_t  ngx_http_uwsgi_cache_headers[] = {
@@ -1995,7 +2002,8 @@ ngx_http_uwsgi_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
         conf->params_source = prev->params_source;
     }
 
-    rc = ngx_http_uwsgi_init_params(cf, conf, &conf->params, NULL);
+    rc = ngx_http_uwsgi_init_params(cf, conf, &conf->params,
+                                    ngx_http_uwsgi_headers);
     if (rc != NGX_OK) {
         return NGX_CONF_ERROR;
     }