add $VERSION to B::Op_private

Since this is an auto-generated .pm file, set $VERSION to the perl
version (i.e. 5.mmmnnn), the same trick that Config.pm does.

Since regen scripts are usually executed by a different perl, and the
current perl may not be built yet, don't rely on $]; instead get the
info from patchlevel.h. Add a new sub for this purpose, perl_version(),
to regen/regen_lib.pl. (I feel that such a routine should already exist
somewhere, but I couldn't find such a beastie.)
perldelta: mention the op_private work