bpo-35134: Add Include/cpython/fileutils.h header file (GH-18493)

Move CPython C API from Include/fileutils.h into a new
Include/cpython/fileutils.h header file which is included by
Include/fileutils.h.

Exclude the following private symbols from the limited C API:

* _Py_error_handler
* _Py_GetErrorHandler()
* _Py_DecodeLocaleEx()
* _Py_EncodeLocaleEx()