manager: allow assignment of properties on target/swap/device units

manager: allow assignment of properties on target/swap/device units

E.g. Documentation or Markers could apply to any unit type. This already worked
partially, because a direct dbus call could be made:

After rebuild with the patch, but before the manager has been restarted:
$ build/systemctl --user set-property dev-zram0.swap Markers=+needs-restart
$ build/systemctl --user show -p Markers dev-zram0.swap
Markers=needs-restart

I noticed that that the rpm unit restart helper was throwing errors for target
units. We should just let the Markers be set for those too, even if it doesn't
do anything in the end. This way we don't need to special-case by unit type.