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: 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