Manage logind's LockedHint

[Steven Allen]

Logind has a per-session LockedHint property. It would be nice if xss-lock could set/unset this on lock/unlock so other programs can tell if the screen locker is active (by listening for changes to this property).

Note: As far as I can tell, this feature isn't fully documented yet. The PR that introduced it is here: https://github.com/systemd/systemd/pull/3238

[Tomáš Janoušek]

This seems to have been implemented here: https://github.com/xdbob/xss-lock/compare/locked_hint