bpo-36763: Rework _PyInitError API (GH-13031)

* Remove _PyInitError.user_err field and _Py_INIT_USER_ERR() macro:
  use _Py_INIT_ERR() instead. _Py_ExitInitError() now longer calls
  abort() on error: exit with exit code 1 instead.
* Add _PyInitError._type private field.
* exitcode field type is now unsigned int on Windows.
* Rename prefix field to _func.
* Rename msg field to err_msg.