nagios: various improvements

* structured config for main config file allows to launch nagios in
debug mode without having to write the whole config file by hand
* build time syntax check
* all options have types, one more example
* I find it misleading that the main nagios config file is linked in
/etc but that if you change the link in /etc/ and restart nagios, it
has no effect. Have nagios use /etc/nagios.cfg
* fix paths in example nagios config files, which allows to reuse it:
  services.nagios.objectDefs =
   (map (x: "${pkgs.nagios}/etc/objects/${x}.cfg")
   [ "templates" "timeperiods" "commands" ]) ++ [ ./main.cfg ]
* for the above reason, add mailutils to default plugins

Co-Authored-By: Aaron Andersen <aaron@fosslib.net>