Clarify that this type only operates on units in /etc/systemd/system.
Also, when state=present, it is not always true that the type is
"installed, enabled and started"
* __systemd_unit: Move systemctl detection to manifest
* __systemd_unit: Restart the unit if inactive
Until now, the --restart parameter caused the unit to be restarted
only when the unit file has changed. This commit modifies --restart
behavior so that the unit is also restarted when the unit is inactive.
* __systemd_unit: Do not create unit file when source is empty