In preparation for accessing it in the screenshield, factor out
common code for ConsoleKit and Systemd.
Also, clean up ConsoleKit manager, as the daemon is required in
a non systemd installation. In particular:
- We allow it to be autostarted at session startup (or really,
we expect it to be already there, started by GDM during session
opening).
- We no longer silently assume that the session is active if
it can't start.
https://bugzilla.gnome.org/show_bug.cgi?id=682096
