Allow to remove the version string in HTTP responses.

Normally Unit responds to HTTP requests by including a header like

  Server: Unit/1.30.0

however it can sometimes be beneficial to withhold the version
information and in this case just respond with

  Server: Unit

This patch adds a new "settings.http" boolean option called
server_version, which defaults to true, in which case the full version
information is sent. However this can be set to false, e.g

  "settings": {
      "http": {
          "server_version": false
      }
  },

in which case Unit responds without the version information as the
latter example above shows.

Link: <https://www.ietf.org/rfc/rfc9110.html#section-10.2.4>
Closes: <https://github.com/nginx/unit/issues/158>
Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_router.c

@@ -1519,6 +1519,12 @@ static nxt_conf_map_t  nxt_router_http_conf[] = {
         NXT_CONF_MAP_INT8,
         offsetof(nxt_socket_conf_t, log_route),
     },
+
+    {
+        nxt_string("server_version"),
+        NXT_CONF_MAP_INT8,
+        offsetof(nxt_socket_conf_t, server_version),
+    },
 };
 
 
@@ -1927,6 +1933,8 @@ nxt_router_conf_create(nxt_task_t *task, nxt_router_temp_conf_t *tmcf,
             skcf->proxy_send_timeout = 30 * 1000;
             skcf->proxy_read_timeout = 30 * 1000;
 
+            skcf->server_version = 1;
+
             skcf->websocket_conf.max_frame_size = 1024 * 1024;
             skcf->websocket_conf.read_timeout = 60 * 1000;
             skcf->websocket_conf.keepalive_interval = 30 * 1000;