The PyOS_AfterFork_Child() function now pass a 'runtime' parameter to
subfunctions.

* Fix _PyRuntimeState_ReInitThreads(): use the correct memory allocator
* Add runtime parameter to _PyRuntimeState_ReInitThreads(),
  _PyGILState_Reinit() and _PyInterpreterState_DeleteExceptMain()
* Move _PyGILState_Reinit() to the internal C API.