Make mdadm scripts interested in mdmonitor.service (Closes: ##1110746)

The mdadm package in forky does not ship mdadm.service (which it did when the mdadm script was transferred to this package), meaning that the init script isn't installed any more. mdmonitor.service looks to be the replacement.

The same problem applies to the mdadm-waitidle init script (where mdadm-waitidle.service appears to have been replaced by mdadm.shutdown, which is not in /lib/systemd/system).

Lintian objects to multiple mappings on the same service; rather than overriding that, have one trigger on mdmonitor.service, the other on mdmon@.service, which should ensure they're installed when mdadm is installed in forky or later.

I expect this to need a stable update, but it needs fixing in unstable first.