See http://www.lambdacs.com/newsgroup/FAQ.html#Q120
and "man pthread_atfork" for background.

I don't use a pthread_atfork handler here - the explicit
approach is more portable.  However calling fork() from
C extensions could still cause trouble.

This patch causes the child to create a new interpreter
lock after doing a fork.  It would be nice to deallocate
the old lock with PyThread_free_lock, but this does some
unwanted error-checking in addition to deallocating the
lock.  So I waste a little memory instead.  To really do
this cleanly one could add a new PyThread_reset_lock function to all the thread_*.h files, and use that instead.