Create new md devices consistently

Create new md devices consistently

 Creating a new MD device with the name 'd-0' results in some
 unexpected behavior, since mdadm sees that '-0' is a
 non-negative integer and therefore makes a "partitionable"
 device (/dev/md_d0).  This is not the expected behavior,
 since the documentation mentions 'dN' several places, and a
 reboot brings it up as /dev/md/d-0.  Make this consistent
 by ensuring that the character immediately following 'd' is
 a digit during creation.

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