Make sure "mdmon" doesn't get called "@dmon".

The Anaconda installer (via its "loader" program) will try to kill
many processes at shutdown, but not "mdmon".

However when mdadm runs mdmon in the Anaconda environment, mdmon
sets argv[0][0] to '@' resulting in "@dmon" which confuses
"loader".

So change mdadm to set argv[0] to a path so that mdmon becomes e.g.
  "@usr/sbin/mdmon"
which "loader" will recognise as being "mdmon".

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