This commits add the bulk of the systemd-logind integration code, but does
not hook it up yet other then calling its init and fini functions, which
don't do that much.

Note the configure bits check for udev since systemd-logind use will only be
supported in combination with udev. Besides that it only checks for dbus
since all communication with systemd-logind is happening over dbus, so
no further libs are needed.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>