improve systemd-login support

Taken from
https://git.gnome.org/browse/gnome-screensaver/commit/?id=5c692be1ae2ea27dff2b3d54a231d8539ca8a545
http://bugzilla.gnome.org/show_bug.cgi?id=669787

https://git.gnome.org/browse/gnome-screensaver/commit/?id=c59c73d0939558eab83730c5a30bea9ef6e6226b

https://git.gnome.org/browse/gnome-screensaver/commit/?id=1a3c239cf88c533be1e489147d621cd9738e203f

https://git.gnome.org/browse/gnome-screensaver/commit/?id=fd353244020cf53ce58f4abf69a5ec7893ca1fd1
http://bugzilla.gnome.org/show_bug.cgi?id=696264