The nmdm code enforces a number between the 'nmdm' and 'A|B' portions
of the device name. This is then used as a unit number, and sprintf'd
back into the tty name. If leading zeros were used in the name,
the created device name is different than the string used for the
clone-open (e.g. /dev/nmdm0001A will result in /dev/nmdm1A).
Since unit numbers are no longer required with the updated tty
code, there seems to be no reason to force the string to be a
number. The fix is to allow an arbitrary string between
'nmdm' and 'A|B', within the constraints of devfs names. This allows
all existing user of numeric strings to continue to work, and also
allows more meaningful names to be used, such as bhyve VM names.
Tested on amd64, i386 and ppc64.
Reported by: Dave Smith
PR: 192281
Reviewed by: neel, glebius
Phabric: D729
MFC after: 3 days
f30a88f9b1