nagios: significant upgrades

 - Upgrade Nagios Core to 4.x
 - Expose mainConfigFile and cgiConfigFile in module for finer
   configuration control.
 - Upgrade Plugins to 2.x
 - Remove default objectDefs, which users probably want to customize.
 - Systemd-ify Nagios module and simplify directory structure
 - Upgrade Nagios package with more modern patch, and ensure the
   statedir is set to /var/lib/nagios

Signed-off-by: Austin Seipp <aseipp@pobox.com>