live: support systemctl to enable *dm

systemd is a tricky beast: getting it to actually launch gdm
required considerable effort on the part of stupid myself.

The relevant links:
* https://wiki.archlinux.org/index.php/Systemd#Running_DEs_under_systemd
* http://unix.stackexchange.com/a/35626/15296

Still looks pretty dirty, and there's some duplication
with a hook in features.in/slinux as well.

features.in/live/live/image-scripts.d/03services

@@ -1,10 +1,28 @@
 #!/bin/sh
 # see also m-p-d::profiles/live/image-scripts.d/init3-services
 
-[ -x /sbin/chkconfig ] || exit 0
+[ -x /sbin/chkconfig -o -x /bin/systemctl ] || exit 0
+
+switch() {
+	case "$2" in
+	on)
+		cc=on; sc=enable;;
+	off)
+		cc=off; sc=disable;;
+	esac
+
+	{
+		[ ! -x /bin/systemctl ] || /bin/systemctl $sc $1.service
+		[ ! -x /sbin/chkconfig ] || /sbin/chkconfig $1 $cc
+	} 2>/dev/null
+}
 
 ENABLE="
 dm
+gdm
+kdm
+wdm
+prefdm
 alteratord
 livecd-evms
 livecd-fstab
@@ -38,6 +56,6 @@ update_wms
 xinetd
 "
 
-for i in $ENABLE;  do chkconfig $i on  2>/dev/null; done
-for i in $DISABLE; do chkconfig $i off 2>/dev/null; done
+for i in $ENABLE;  do switch $i on;  done
+for i in $DISABLE; do switch $i off; done
 :