message: fix nydusd daemon process double wait and change p.Kill() to
  p.Signal(syscall.SIGTERM) when destroy nydusd daemon process
diff: @@ -121,8 +121,7 @@ func (m *Manager) StartDaemon(d *daemon.Daemon) error {
  return err
  }
  d.Pid = cmd.Process.Pid
  - // make sure to wait after start
  - go cmd.Wait()
  + // process wait when destroy daemon and kill process
  return nil
  }
  @@ -182,7 +181,7 @@ func (m *Manager) DestroyDaemon(d *daemon.Daemon) error {
  if err != nil {
  return err
  }
  - err = p.Kill()
  + err = p.Signal(syscall.SIGTERM)
  if err != nil {
  return err
  }
  