check_mailq: try to autodetect which mailq implementation we are using

This is done by looking at some common directories and files each
MTA installs on the system. If no known file is found, the old default
sendmail is used. Of course this still can be overridden by -M.
check_mailq: document autodetection in the usage output
check_mailq: add $mailq to check output, so it is easily visible what was autodetected

Closes: #1242
NEWS: Adding check_mailq auto detection