bpo-36763: Add _PyPreConfig._config_init (GH-13481)

* _PyPreConfig_GetGlobalConfig() and  _PyCoreConfig_GetGlobalConfig()
  now do nothing if the configuration was not initialized with
  _PyPreConfig_InitCompatConfig() and _PyCoreConfig_InitCompatConfig()
* Remove utf8_mode=-2 special case: use utf8_mode=-1 instead.
* Fix _PyPreConfig_InitPythonConfig():

  * isolated = 0 instead of -1
  * use_environment = 1 instead of -1

* Rename _PyConfig_INIT to  _PyConfig_INIT_COMPAT
* Rename _PyPreConfig_Init() to _PyPreConfig_InitCompatConfig()
* Rename _PyCoreConfig_Init() to _PyCoreConfig_InitCompatConfig()
* PyInterpreterState_New() now uses _PyCoreConfig_InitPythonConfig()
  as default configuration, but it's very quickly overriden anyway.
* _freeze_importlib.c uses _PyCoreConfig_SetString() to set
  program_name.
* Cleanup preconfig_init_utf8_mode(): cmdline is always non-NULL.
diff --git a/Python/pathconfig.c b/Python/pathconfig.c
index 3d9d3b1..bbf29b2 100644
--- a/Python/pathconfig.c
+++ b/Python/pathconfig.c
@@ -383,7 +383,7 @@
 
     _PyInitError err;
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitCompatConfig(&config);
 
     err = _PyCoreConfig_Read(&config);
     if (_Py_INIT_FAILED(err)) {