Ability to discover disabled systemd units

Currently systemd.unit.discovery only discovers system units which are either enabled (or started once). In some use-cases this is undesired.

It is suggested (without modifying current behavior) to implement new optional parameter <state> parameter with possible values:

• enabled (default)
• disabled
• all

Implementation-wise this is possible by making a call to DBus method ListUnitFiles (currently only ListUnits is used). The difference between two calls can be seen by comparing "systemctl list-units" and "systemctl list-unit-files" outputs.