added a vsnprintf() implementation from cvslock. See the notes on the

added a vsnprintf() implementation from cvslock. See the notes on the
license at the top of lib/snprintf.c

I've always been slightly uneasy about our half-baked vslprintf()
implementation and the risks on platforms that don't have vsnprintf()
so when I saw this code in another GPLd package I wanted it for Samba.
(This used to be commit e2cb50af45be6683d02ab48a6648816ee3d52ab1)