If the new name start with '/', cur_dir is not prepened any more, so that
the current working directory is respected. And extend the test cases for
rename to cover this functionality.
This change scans for '.', '..' and multiple '/' and normalizes the new
path name. If the resulting path does not exist, an error is raised.
Non-existing interim path elements are ignored if they are removed during
normalization.
This fixes the bug, that stat(filename) would not consider the current
working directory. So if e.g. the cwd is "lib", then stat("main.py") would
return the info for "/main.py" instead of "/lib/main.py".
