"keep the C function"

Hum, currently, Python3 only has a *private* function called _Py_SetFileSystemEncoding() which can only be called after _Py_InitializeEx() (because it relies on the codecs API). If you consider that there is a real use case, we should create a function to set the filesystem encoding, function that should (have to?) be called before Py_InitializeEx().

I still think that Python knows better than the application how to set the encoding (when, how to choose it, etc.).