disallow moving up and down the path with .. and .

Before using the path, compare it with the cleaned-up
version of itself (given by os.path.normpath). It it's
different, so eitehr there's a ../ or /./ or // (double slash),
deny access and throw a 400 bad request.

The storage layer gives a MalformedPathException, the cdmi layer
hands this to the client as 400.