'Create a subdirectory in the tempdir and remember it so as to call
shutdown() on it before attempting to clean up.
@rtype: _Dir instance'

def subdir(self, dirname):

    ffn = os.path.join(self.name, dirname)
    sd = _Dir(ffn, self.cleanup)
    self.register_subdir(sd)
    make_dirs(sd.name)
    return sd
