Make $$ writable, but still magical

Make $$ writable, but still magical

This commit makes $$ writable again, as it was in 5.6, while preserv-
ing the magical pid-fetching added recently (post-5.14.0) by com-
mit 0e219455.

It does this by following Aristotle Pagaltzis’ brilliant suggestion in
<20110609145148.GD8471@klangraum.plasmasturm.org>; namely, to store
the PID in magic when $$ is written to, so that get-magic can detect
whether a fork() has occurred and reset $$ accordingly.  This makes it
seem as though the fork() code sets $$ itself (which it used to before
0e219455), while even working when C code outside of perl’s control
calls fork().

This restores compatibility with DBIx::Connector and PPerl.