check_mailq.pl: separate submission queue

check_mailq.pl ignores the separate submission queue used in (modern?) sendmail
implementations.

For the queue output below with one message in the submission queue and no
messages in the transport queue, check_mailq.pl reports zero messages in the
queue because the request count from the last queue always overwrites previous
queues. If the sendmail MTA isn't running or has become wedged, messages will
sit in the submission queue forever.

The attached patch fixes this in a backwards compatible way (i.e., it shouldn't
break any of the currently supported formats).
--
Just turning attached patch of github issue #972 into a push request.
(Closes #972)