Implement 'split -d' which allows a numeric suffix instead of an
alphabetic one.

PR:		bin/116209
Submitted by:	Marcin Gryszkalis <mg@fork.pl> (adapted from)
Reviewed by:	will
MFC after:	1 week
Remove the commented out code I just committed. If we need it, I'll
bring it back uncommented our, or rewrite it.
As requested, move this back to opt-in and list my boards.