check_mailq: Fixing nullmailer regex

[waja]

In Debian we got reported this issue: https://bugs.debian.org/1019406

it seems that the output from nullmailer's mailq has changed format since
that portion was implemented. i noticed that it was not alerting on a
host w/many messages in its queue:

% mailq | wc -l
12488
% check_mailq -M nullmailer -w 5 -c 10
OK: nullmailer mailq (0) is below threshold (5/10)|unsent=0;5;10;0

attached is a patch that updates the format expected in the nullmailer
mailq output. the regex is a little more flexible and less specific
than the previous version.

[RincewindsHat]

Compared that to https://github.com/bruceg/nullmailer/blob/master/test/tests/mailq . Looks good to me.