systemd: Update to 212

Note that systemd no longer depends on dbus, so we're rid of the
cyclic dependency problem between systemd and dbus.

This commit incorporates from wkennington's systemd branch
(203dcff45002a63f6be75c65f1017021318cc839,
1f842558a95947261ece66f707bfa24faf5a9d88).