Issue 6923: Need pthread_atfork-like functionality in CPython

In order to properly handle multiple threads and fork()-calls from C code 
(rather than os.fork()), Python should provide pthread_atfork()-like 
functionality: a function to call before a fork, to acquire any locks 
that need to be acquired, and a pair of functions to call after the fork, 
in the parent and the child, to release or re-allocate locks. The 
acquisitions should be re-entrant and should not require an existing 
threadstate. Also see <http://bugs.python.org/issue1590864>.