logind: Allow active sessions to always shutdown by default

Currently local user are being asked for an admin  password when another
user is logged into the system. This does not make sense as the user has
the power to shut down the system anyway regardless if he/she knows the
password or not (by pulling the plug, battery or whatever).

So only require the admin authentification for remote sessions.

https://bugzilla.gnome.org/show_bug.cgi?id=693385