bpo-32030: Make _PySys_AddXOptionWithError() private (GH-10236)

Make _PySys_AddXOptionWithError() and _PySys_AddWarnOptionWithError()
functions private again. They are no longer needed to initialize Python:
_PySys_EndInit() is now responsible to add these options instead.

Moreover, PySys_AddWarnOptionUnicode() now clears the exception on
failure if possible.