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
utils.pm: use FindBin instead of awk to find the path to

'use lib utils.pm' is not valid Perl syntax:
 Bareword "utils" not allowed while "strict subs" in use at plugins-scripts/check_ircd.pl line 52.
 Bareword "pm" not allowed while "strict subs" in use at plugins-scripts/check_ircd.pl line 52.

This makes it impossible to use the plugins directly from the git tree,
e.g. while hacking on them.

Using FindBin::Bin as the library path allows that, while preserving
the original behaviour of adding the libexec path when the plugin is
properly installed.
subst.in: remove the led() function

It was only used to replace the 'use lib utils.pm' with the proper
libexec dir, which is now solved by using Perl's FinBin.

Closes #1241