This fixes a serious bug on NixOS with swraid where mdadm arrays weren't
properly stopped on shutdown. Rather than fixing the unit by adding
`Before=final.target` we completely move to upstream units, which uses
systemd shutdown hooks instead. This also drives down maintenance costs
for us.
Also add required systemd services for starting/stopping mdmon.
Closes#13447.
abbradar: fixed `mdadmShutdown` service name according to de facto conventions.