* Rename _Py_GetLocaleEncoding() to _Py_GetLocaleEncodingObject()
* Add _Py_GetLocaleEncoding() which returns a wchar_t* string to
  share code between _Py_GetLocaleEncodingObject()
  and config_get_locale_encoding().
* _Py_GetLocaleEncodingObject() now decodes nl_langinfo(CODESET)
  from the current locale encoding with surrogateescape,
  rather than using UTF-8.

* bpo-42146: Unify cleanup in subprocess_fork_exec()

Also ignore errors from _enable_gc():
* They are always suppressed by the current code due to a bug.
* _enable_gc() is only used if `preexec_fn != None`, which is unsafe.
* We don't have a good way to handle errors in case we successfully
  created a child process.

Co-authored-by: Gregory P. Smith <greg@krypto.org>

* Add a new _locale._get_locale_encoding() function to get the
  current locale encoding.
* Modify locale.getpreferredencoding() to use it.
* Remove the _bootlocale module.

_io.TextIOWrapper no longer calls getpreferredencoding(False) of
_bootlocale to get the locale encoding, but calls
_Py_GetLocaleEncoding() instead.

Add config_get_fs_encoding() sub-function. Reorganize also
config_get_locale_encoding() code.

The last GC collection is now done before clearing builtins and sys
dictionaries. Add also assertions to ensure that gc.collect() is no
longer called after _PyGC_Fini().

Pass also the tstate to PyInterpreterState_Clear() to pass the
correct tstate to _PyGC_CollectNoFail() and _PyGC_Fini().

Move private _PyGC_CollectNoFail() to the internal C API.

Remove the private _PyGC_CollectIfEnabled() which was just an alias
to the public PyGC_Collect() function since Python 3.8.

Rename functions:

* collect() => gc_collect_main()
* collect_with_callback() => gc_collect_with_callback()
* collect_generations() => gc_collect_generations()

IRIX code was slowy removed in Python 2.4 (--with-sgi-dl), Python 3.3
(Irix threads), and Python 3.7.

Move the _PyLong_GetOne() call outside the fast-path loop.