PyOS_AfterFork_Child() uses PyStatus (GH-20596)

PyOS_AfterFork_Child() helper functions now return a PyStatus:
PyOS_AfterFork_Child() is now responsible to handle errors.

* Move _PySignal_AfterFork() to the internal C API
* Add #ifdef HAVE_FORK on _PyGILState_Reinit(), _PySignal_AfterFork()
  and _PyInterpreterState_DeleteExceptMain().