Rename _PyInterpreterState_GET_UNSAFE() to _PyInterpreterState_GET()
for consistency with _PyThreadState_GET() and to have a shorter name
(help to fit into 80 columns).

Add also "assert(tstate != NULL);" to the function.

pycore_pystate.h no longer includes pycore_interp.h:
it's now included explicitly in files accessing PyInterpreterState.

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.

Assume gettimeofday exists and takes two arguments.

* Define THREAD_STACK_SIZE for AIX to pass default recursion limit test

This changeset increases the default size of the stack
for threads on macOS to the size of the stack
of the main thread and reenables the relevant
recursion test.