Speed up screenlocking on systems with logind

Since logind is able to handle the screen locking, we're now prefering
to use it, since it is handling the locking much more simple and reliable
than our attempts to support multiple desktop environments.

Change-Id: Iaaf01e5c6c612178f207e3acfc877d30e0a09ec8
Bug-Url: https://bugzilla.redhat.com/1097390
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>