Add --prefer option for --detail and --monitor

Add --prefer option for --detail and --monitor

Both --detail and --monitor can report the names of member
devices on an array, and do so by searching /dev and finding
the shortest name that matches.

If
   --prefer=foo
is given, they will instead prefer a name that contain /foo/.
So
   mdadm --detail /dev/md0 --prefer=by-path

will list the component devices via their /dev/disk/by-path/xxx
names.

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