Isolation: added "automount" option.

Now it's possible to disable default bind mounts of
languages by setting:

  {
      "isolation": {
           "automount": {
               "language_deps": false
           }
     }
  }

In this case, the user is responsible to provide a "rootfs"
containing the language libraries and required files for
the application.

src/nxt_main_process.c

@@ -1290,6 +1290,8 @@ nxt_main_port_modules_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
                 goto fail;
             }
 
+            mnt->builtin = 1;
+
             ret = nxt_conf_map_object(rt->mem_pool, value,
                                       nxt_app_lang_mounts_map,
                                       nxt_nitems(nxt_app_lang_mounts_map), mnt);