bpo-36142: _PyPreConfig_Read() sets LC_CTYPE (GH-12188)

* _PyPreConfig_Read() now sets temporarily LC_CTYPE to the user
  preferred locale, as _PyPreConfig_Write() will do permanentely.
* Fix _PyCoreConfig_Clear(): clear run_xxx attributes
* _Py_SetArgcArgv() doesn't have to be exported
* _PyCoreConfig_SetGlobalConfig() no longer applies preconfig