Fix trusted path

When configuring the plugins you have the option to specify a trusted path with --with-trusted-path. This option seems to have been deprecated at some point for unknown reasons and had typically had no affect. This commit makes this option work again for those who have a desire to use it. There should be no affect on those who have not chosen to specify a trusted path.
travis-ci: Don't use Clang

Currently, there doesn't seem to be a way to configure Travis CI to omit
the Clang build on the coverity/* branches.

See: https://github.com/travis-ci/travis-ci/issues/1975
check_swap: Fix the plugin name that appears in the comment

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>
Rename configure.in to configure.ac

The old name has been deprecated years ago.  The Autoconf documentation
says:

| Previous versions of Autoconf promoted the name configure.in, which is
| somewhat ambiguous (the tool needed to process this file is not
| described by its extension), and introduces a slight confusion with
| config.h.in and so on (for which `.in' means "to be processed by
| configure").  Using configure.ac is now preferred.

[ https://www.gnu.org/software/autoconf/manual/autoconf.html ]
configure.ac: Support --disable-maintainer-mode

This

| allows you to choose whether the so called "rebuild rules" should be
| enabled or disabled.  With AM_MAINTAINER_MODE([enable]), they are
| enabled by default, otherwise they are disabled by default.  In the
| latter case, if you have AM_MAINTAINER_MODE in configure.ac, and run
| `./configure && make', then make will *never* attempt to rebuild
| configure, Makefile.ins, Lex or Yacc outputs, etc.  I.e., this
| disables build rules for files that are usually distributed and that
| users should normally not have to update.
|
| The user can override the default setting by passing either
| `--enable-maintainer-mode' or `--disable-maintainer-mode' to
| configure.
|
| People use AM_MAINTAINER_MODE either because they do not want their
| users (or themselves) annoyed by timestamps lossage (see CVS), or
| because they simply can't stand the rebuild rules and prefer running
| maintainer tools explicitly.

[ https://www.gnu.org/software/automake/manual/automake.html ]
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
travis-ci: Remove gawk to build against mawk
Merge remote-tracking branch 'awiddersheim/fix_trusted_path'

* awiddersheim/fix_trusted_path:
  Fix trusted path

Conflicts:
	plugins-scripts/check_ntp.pl
	plugins-scripts/subst.in

Closes #1212.
Replace "plugins-scripts/subst.in" foo

Remove the buggy and complex awk(1) magic in "plugins-scripts/subst.in"
in favor of simple sed(1) substitutions.

The plugins in the "plugins-scripts" directory now always use the PATH
specified via "./configure --trusted-path", or the default PATH
hard-coded in "configure.ac".

Fixes #1242.
configure.ac: Remove unused AC_PATH_PROG() calls