semplice-logout: switch to logind.

Sometimes users are unable to shutdown the "conventional" way because logind has taken over ConsoleKit.
This commit switches everything to logind (including the Suspend and Hibernate actions previously handled by UPower).
Merge branch 'master' into debian
debian/control: removed dependency on consolekit and upower, added dependency on systemd (or systemd-shim) and python-t9n.
6.1.0-1