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.

Python/thread_pthread.h

@@ -693,6 +693,26 @@ PyThread_release_lock(PyThread_type_lock lock)
 
 #endif /* USE_SEMAPHORES */
 
+int
+_PyThread_at_fork_reinit(PyThread_type_lock *lock)
+{
+    PyThread_type_lock new_lock = PyThread_allocate_lock();
+    if (new_lock == NULL) {
+        return -1;
+    }
+
+    /* bpo-6721, bpo-40089: The old lock can be in an inconsistent state.
+       fork() can be called in the middle of an operation on the lock done by
+       another thread. So don't call PyThread_free_lock(*lock).
+
+       Leak memory on purpose. Don't release the memory either since the
+       address of a mutex is relevant. Putting two mutexes at the same address
+       can lead to problems. */
+
+    *lock = new_lock;
+    return 0;
+}
+
 int
 PyThread_acquire_lock(PyThread_type_lock lock, int waitflag)
 {