bpo-34523: Add _PyCoreConfig.filesystem_encoding (GH-8963)

_PyCoreConfig_Read() is now responsible to choose the filesystem
encoding and error handler. Using Py_Main(), the encoding is now
chosen even before calling Py_Initialize().

_PyCoreConfig.filesystem_encoding is now the reference, instead of
Py_FileSystemDefaultEncoding, for the Python filesystem encoding.

Changes:

* Add filesystem_encoding and filesystem_errors to _PyCoreConfig
* _PyCoreConfig_Read() now reads the locale encoding for the file
  system encoding.
* PyUnicode_EncodeFSDefault() and PyUnicode_DecodeFSDefaultAndSize()
  now use the interpreter configuration rather than
  Py_FileSystemDefaultEncoding and Py_FileSystemDefaultEncodeErrors
  global configuration variables.
* Add _Py_SetFileSystemEncoding() and _Py_ClearFileSystemEncoding()
  private functions to only modify Py_FileSystemDefaultEncoding and
  Py_FileSystemDefaultEncodeErrors in coreconfig.c.
* _Py_CoerceLegacyLocale() now takes an int rather than
  _PyCoreConfig for the warning.
diff --git a/Modules/main.c b/Modules/main.c
index 3a15b2b..2e9e23b 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -1339,7 +1339,7 @@
          */
         if (config->coerce_c_locale && !locale_coerced) {
             locale_coerced = 1;
-            _Py_CoerceLegacyLocale(config);
+            _Py_CoerceLegacyLocale(config->coerce_c_locale_warn);
             encoding_changed = 1;
         }