bpo-36763: Add _PyInitError functions (GH-13395)

* Add _PyInitError functions:

  * _PyInitError_Ok()
  * _PyInitError_Error()
  * _PyInitError_NoMemory()
  * _PyInitError_Exit()
  * _PyInitError_IsError()
  * _PyInitError_IsExit()
  * _PyInitError_Failed()

* frozenmain.c and _testembed.c now use functions rather than macros.
* Move _Py_INIT_xxx() macros to the internal API.
* Move _PyWstrList_INIT macro to the internal API.