Check for logind, not for systemd

It is possible to build systemd without logind, in which case
/sys/fs/cgroup/systemd would still exist. Check for /run/systemd/seats instead,
as recommended by systemd upstream.

For details, see:
<https://mail.gnome.org/archives/desktop-devel-list/2013-March/msg00092.html>

Drop the now unnecessary linking against libsystemd-daemon.

https://bugzilla.gnome.org/show_bug.cgi?id=696264
Merge remote-tracking branch 'upstream/master'