message: udp_connection: another cleanup fix
  Somehow, shutdown doesn't properly work on Mac, and we need to call
  close as well.
diff: @@ -79,17 +79,18 @@ DroneLink::ConnectionResult UdpConnection::stop()
  {
  _should_exit = true;
  - // This interrupts a recv/recvfrom call.
  + // This should interrupt a recv/recvfrom call.
  shutdown(_socket_fd, SHUT_RDWR);
  + // But on Mac, closing is also needed to stop blocking recv/recvfrom.
  + close(_socket_fd);
  +
  if (_recv_thread) {
  _recv_thread->join();
  delete _recv_thread;
  _recv_thread = nullptr;
  }
  - close(_socket_fd);
  -
  // We need to stop this after stopping the receive thread, otherwise
  // it can happen that we interfere with the parsing of a message.
  stop_mavlink_receiver();
  @@ -152,7 +153,9 @@ void UdpConnection::receive(UdpConnection *parent)
  }
  if (recv_len < 0) {
  - Debug() << "recvfrom error: " << strerror(errno);
  + // This happens on desctruction when close(_socket_fd) is called,
  + // therefore be quiet.
  + //Debug() << "recvfrom error: " << strerror(errno);
  continue;
  }
  