bpo-40910: Export Py_GetArgcArgv() function (GH-20721)

Export explicitly the Py_GetArgcArgv() function to the C API and
document the function. Previously, it was exported implicitly which
no longer works since Python is built with -fvisibility=hidden.

* Add PyConfig._orig_argv member.
* Py_InitializeFromConfig() no longer calls _PyConfig_Write() twice.
* PyConfig_Read() no longer initializes Py_GetArgcArgv(): it is now
  _PyConfig_Write() responsibility.
* _PyConfig_Write() result type becomes PyStatus instead of void.
* Write an unit test on Py_GetArgcArgv().