This reverts commit 066e5b1a917ec2134e8997d2cadd815724314252.

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.

Remove daemon threads from :mod:`concurrent.futures` by adding
an internal `threading._register_atexit()`, which calls registered functions
prior to joining all non-daemon threads. This allows for compatibility
with subinterpreters, which don't support daemon threads.

If fork was not called by a thread spawned by threading.Thread,
threading._after_fork() now creates a _MainThread instance for
_main_thread, instead of a _DummyThread instance.

* Use the 'p' format unit instead of manually called PyObject_IsTrue().
* Pass boolean value instead 0/1 integers to functions that needs boolean.
* Convert some arguments to boolean only once.

This reverts commit d3dcc92778807ae8f7ebe85178f36a29711cd478.

Add threading.Thread.__del__() method to ensure that the thread state
lock is removed from the _shutdown_locks list when a thread
completes.

Rephrase and clarify that "the entire Python program exits when only daemon threads are left". This matches the documentation at https://docs.python.org/3/library/threading.html#thread-objects.