logind: fix check for multiple sessions

logind: fix check for multiple sessions

The "$action-multiple-sessions" polkit actions are defined as
"$action while other users are logged in". To me this implies that the
following sessions should not count:
 - greeter sessions
 - user sessions belonging to the same user as the one who's asking

Not sure how to treat class SESSION_LOCK_SCREEN. I never have these.
I just ignore every class that's not SESSION_USER.

https://bugzilla.redhat.com/show_bug.cgi?id=814424