bpo-36142: Add _PyPreConfig.utf8_mode (GH-12174)

* Move following fields from _PyCoreConfig to _PyPreConfig:

  * coerce_c_locale
  * coerce_c_locale_warn
  * legacy_windows_stdio
  * utf8_mode

* _PyPreConfig_ReadFromArgv() is now responsible to choose the
  filesystem encoding
* _PyPreConfig_Write() now sets the LC_CTYPE locale