C<foreach my $x ...> in pseudo-fork()ed process may diddle

C<foreach my $x ...> in pseudo-fork()ed process may diddle
parent's memory; fix it by keeping track of the actual pad
offset rather than a raw pointer (this change is probably also
relevant to non-ithreads case to avoid fallout from reallocs of
the pad array, but is currently only enabled for the ithreads
case in the interests of minimal disruption to existing "well
tested" code)

p4raw-id: //depot/perl@7858