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_conf_validation.c

@@ -347,6 +347,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_http_members[] = {
     }, {
         .name       = nxt_string("log_route"),
         .type       = NXT_CONF_VLDT_BOOLEAN,
+    }, {
+        .name       = nxt_string("server_version"),
+        .type       = NXT_CONF_VLDT_BOOLEAN,
     },
 
     NXT_CONF_VLDT_END