Better handling of systemd parametrised and other special units not listed by list-unit-files.

This allows to query status, start, stop, restart and list units that are not actually provided by unit files. Such units cannot be enabled/disabled and that's why those actions still prefer the "list-unit-files" output over "list-units".

Units that couldn't be handled otherwise include for example mount units and sysvinit compatibility units such as those present on debian systems.

The output of a "service.running ssh" state on a debian wheezy target is:

          ID: ssh
    Function: service.running
      Result: False
     Comment: The named service ssh is not available
     Changes:

after this patch:

          ID: ssh
    Function: service.running
      Result: True
     Comment: The service ssh is already running
     Changes: