bpo-46303: Move fileutils.h private functions to internal C API (GH-30484)

Move almost all private functions of Include/cpython/fileutils.h to
the internal C API Include/internal/pycore_fileutils.h.

Only keep _Py_fopen_obj() in Include/cpython/fileutils.h, since it's
used by _testcapi which must not use the internal C API.

Move EncodeLocaleEx() and DecodeLocaleEx() functions from _testcapi
to _testinternalcapi, since the C API moved to the internal C API.