mdmon: Remove need for KillMode=none

mdmon: Remove need for KillMode=none

mdmon needs to keep running during the switchroot out of (at boot) and
then back into (at shutdown) the initrd.  It runs until a new mdmon
takes over.

Killmode=none is used to achieve this, with the help of --offroot which
sets argv[0][0] to '@' which systemd understands.

This is needed because mdmon is currently run in system-mdmon.slice
which conflicts with shutdown.target so without Killmode=none mdmon
would get killed early in shutdown when system.mdmon.slice is removed.

As described in systemd.service(5), this conflict with shutdown can be
resolved by explicitly requesting system.slice, which is a natural
counterpart to DefaultDependencies=no.

So add that, and also add IgnoreOnIsolate=true to avoid another possible
source of an early death.  With these we no longer need KillMode=none
which the systemd developers have marked as "deprecated".

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>