Add systemd support to debian-edu-restart-services

This is a bit tricky, the radical approach employed by the sysvinit code of stopping everything which is not part of runlevel 1 could only reasonably be emulated by systemctl isolate rescue.target followed by systemctl isolate default.target or employing snapshots. However, that would also kill off a terminal emulator where the command was run from. systemd v254 also provides soft-reboot but as the former solution the difference to a real reboot would be marginal.

This uses a list of service units which was compiled on a main server + ltsp installation. Uses stop and start to force restart reverse-dependencies. It also makes sure that drop in files are recognized. Closes: #1042940