docs: def unpackmar(marfile, destdir):
      """Unpack marfile into destdir"""
      marfile = cygpath(os.path.abspath(marfile))
      nullfd = open(os.devnull, "w")
      try:
          check_call([MAR, '-x', marfile], cwd=destdir,
                     stdout=nullfd, preexec_fn=_noumask)
      except Exception:
          log.exception("Error unpacking mar file %s to %s", marfile, destdir)
          raise
      nullfd.close()