Try stopping MD RAID devices in shutdown too

Try stopping MD RAID devices in shutdown too

Currently the systemd-shutdown command attempts to stop swaps, DM
(crypt, LVM2) and loop devices, but it doesn't attempt to stop MD
RAID devices, which means that if the RAID is set up on crypt,
loop, etc. device, it won't be able to stop those underlying devices.

This code extends the shutdown application to also attempt stopping
the MD RAID devices.

Signed-off-by: Hubert Kario <hubert@kario.pl>