ether_fakeaddr: Use 'b' 's' 'd' for the prefix

This has the advantage of being obvious to sniff out the designated prefix
by eye and it has all the right bits set. Comment stolen from ffec.

I've removed bryanv@'s pending question of using the FreeBSD OUI range --
no one has followed up on this with a definitive action, and there's no
particular reason to shoot for it and the administrative overhead that comes
with deciding exactly how to use it.