bpo-40089: Add _at_fork_reinit() method to locks (GH-19195)

Add a private _at_fork_reinit() method to _thread.Lock,
_thread.RLock, threading.RLock and threading.Condition classes:
reinitialize the lock after fork in the child process; reset the lock
to the unlocked state.

Rename also the private _reset_internal_locks() method of
threading.Event to _at_fork_reinit().

* Add _PyThread_at_fork_reinit() private function. It is excluded
  from the limited C API.
* threading.Thread._reset_internal_locks() now calls
  _at_fork_reinit() on self._tstate_lock rather than creating a new
  Python lock object.

Modules/posixmodule.c

@@ -491,7 +491,8 @@ register_at_forker(PyObject **lst, PyObject *func)
     }
     return PyList_Append(*lst, func);
 }
-#endif
+#endif  /* HAVE_FORK */
+
 
 /* Legacy wrapper */
 void