Consolidate ConsoleKit and Systemd code

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