dma(8): Better integration with our periodic scripts.

* Make dma(8) accept the -Ac option when run as 'mailq'. Also make it
  run as 'hoststat' and 'purgestat' (does nothing in this case).

* Adjust the 440.status-mailq daily periodic script to also check for
  "Mail queue is empty", which is dma(8)'s corresponding text.

Note: We really should be suppressing the checking of the submit queue
      automatically if mailer.conf is set up for dma(8) but I'm not
      sure yet what's the best way.

Reported and tested by Max Herrgard <herrgard@gmail.com>.