bpo-36142: Move command line parsing to coreconfig.c (GH-12123)

* Add _PyCoreConfig_ReadFromArgv() function which parses command line
  options: move code from main.c to coreconfig.c.
* Add _PyCoreConfig_Write() to write the new configuration: coerce
  the LC_CTYPE locale, set Py_xxx global configuration variables,
  etc.
* _PyCoreConfig_ReadFromArgv() now only changes the LC_CTYPE locale
  temporarily. _PyCoreConfig_Write() becomes responsible to set the
  LC_CTYPE locale.
* Add _Py_SetArgcArgv() and _Py_ClearArgcArgv() functions
* Rename many "pymain_xxx()" functions
* Add "const" to some function parameters
* Reorganize main.c to declare functions in the order in which they
  are called.