Add mdmonitor.service systemd unit file.

Add mdmonitor.service systemd unit file.

This systemd unit file runs mdadm in --monitor mode.
It is started by a SYSTEMD_WANTS signal from udev whenever
an md array is started that would benefit from mdadm --monitor.

Commandline arguments can be provided by a script
  /usr/lib/systemd/scripts/mdadm_env.sh
which should write an
  MDADM_MONITOR_ARGS=....
line to /run/sysconfig/mdadm

A script to extra args from SUSE's /etc/sysconfig/mdadm file
is provided.
If no mdadm_env.sh is provided, then args are "--scan" which
requires "mail" or "program" to be set in /etc/mdadm.conf.
I believe this is suitable for Fedora.

Signed-off-by: NeilBrown <neilb@suse.de>