Introducing the shared application port.

This is the port shared between all application processes which use it to pass
requests for processing.  Using it significantly simplifies the request
processing code in the router.  The drawback is 2 more file descriptors per each
configured application and more complex libunit message wait/read code.

src/nxt_port.c

@@ -67,8 +67,6 @@ nxt_port_new(nxt_task_t *task, nxt_port_id_t id, nxt_pid_t pid,
 
         nxt_queue_init(&port->messages);
         nxt_thread_mutex_create(&port->write_mutex);
-        nxt_queue_init(&port->pending_requests);
-        nxt_queue_init(&port->active_websockets);
 
     } else {
         nxt_mp_destroy(mp);