power: Avoid automatic logout in GDM/greeter

In GDM sessions (greeter, initial-setup), it does not make sense to
automatically logout. This can happen if the system wide default is
changed to default to the "logout" action.

Note that we already use the RUNNING_UNDER_GDM environment variable in
the keyboard plugin currently. So doing this is likely sane, even if we
probably want a more elegant strategy to detect whether we are in a
"login" session.