Extra-Opts configuration file location(s)

[Holger Weiß]

In the upcoming Monitoring Plugins release, the Extra-Opts feature will
be enabled by default.  See:

	https://www.monitoring-plugins.org/doc/extra-opts.html

The configuration file used by that feature can be specified on the
command line or via an environment variable.  If neither is done, a list
of default locations is searched.

I'd like to change the environment variable used for this purpose:
Currently, NAGIOS_CONFIG_PATH can be set to a colon-separated list of
directories to be searched.  I think that's overkill and would prefer an
MP_CONFIG_FILE variable that points to a single file path instead.  If
nobody objects, I'll update the code and documentation accordingly.
Either way, we'll continue to support NAGIOS_CONFIG_PATH for
compatibility, at least for a while.

Holger