message: nydusd: fix a bug in handling singleton subcommand
  Let the singleton daemon exits when received SIGINT.
diff: @@ -142,6 +142,7 @@ impl DaemonController {
  fn shutdown(&self) {
  // Marking exiting state.
  self.active.store(false, Ordering::Release);
  + DAEMON_CONTROLLER.set_singleton_mode(false);
  // Signal the `run_loop()` working thread to exit.
  let _ = self.waker.wake();
  