If screen locking is enabled, the screen shield should drop down
on suspend. Currently this is achieved by either explicitly locking
the screen (when selecting "Suspend" from the user menu) or by
relying on g-s-d delaying the suspension enough time for the shield
to get into place (lid close, power button).
Systemd inhibitors offer a safer way to ensure that the screen is
locked before going to sleep, so add a small abstraction for them
in the loginManager - with inhibitors being a systemd-only feature,
the ConsoleKit path only has a dummy implementation.
https://bugzilla.gnome.org/show_bug.cgi?id=686482
8747c0c58d