mdmon: honour --offroot, again

mdmon: honour --offroot, again

commit 3e32ba9d removed support for --offroot, and a9c15847 made
mdmon use @ in argv[0] only when started from initrd.

This breaks mdadm in OpenSUSE 12.3, which starts mdmon from the
root file system and relies on --offroot to work as documented earlier.

Reintroducing --offroot as an undocumented option, as its use is going to
go away soon anyway.

If this can't be applied, it should probably be included as distro-specific
patch if mdadm 3.3 is built for OpenSUSE 12.3. I haven't checked if the
patch is necesary for OpenSUSE Factory, too.

Signed-off-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>