Added extra systemd logic to 3.6

- Backported some changes made/discovered while implementing the 3.6
  version to 3.5, including improved logic (actually used the
  can_{stop|start}_service classes) and support for a custom
  service/call, e.g. isolate, try-restart, et al.
- Uses the ifvarclass to make the commands section a little cleaner
- Uses the 'kept_successful_command' on all explicit systemd actions