bpo-36301: Cleanup preconfig code (GH-12535)

Prepare code to move some _PyPreConfig parameters into _PyPreCmdline.
Changes:

* _PyCoreConfig_ReadFromArgv(): remove preconfig parameter,
  use _PyRuntime.preconfig.
* Add _PyPreCmdline_GetPreConfig() (called by _PyPreConfig_Read()).
* Rename _PyPreCmdline_Init() to _PyPreCmdline_SetArgv()
* Factorize _Py_PreInitializeFromPreConfig() code: add
  pyinit_preinit().
* _PyPreConfig_Read() now sets coerce_c_locale to 2 if it must be
  coerced.
* Remove _PyCoreConfig_ReadPreConfig().
* _PyCoreConfig_Write() now copies updated preconfig into _PyRuntime.