check_mailq: add sudo support

Just turning attached patch of github issue #1099 into a push request
(with some modifications)

"The check_mailq plug-in calls /usr/bin/mailq which, in my environments, requires root access. This is easily accomplished by giving nagios sudo access to /usr/bin/mailq but check_mailq needs to call sudo in order to use those privileges.

I have made a small patch that accomplishes this goal with an additional parameter (-s or --sudo). The user still needs to edit /etc/sudoers to give the nagios user permissions to execute /usr/bin/mailq without a password.

I attached my patch to my original issue report over at the old Nagios plugins tracker here: http://tracker.nagios.org/view.php?id=368"