util: refactor opener - Don't call atomictempfile or nlinks() if the path is malformed (no basename). Let posixfile() raise IOError directly. - atomictempfile already breaks up hardlinks, no need to poke at the file with nlinks() if atomictemp. - No need to copy the file contents to break hardlinks for 'w'rite modes (w, wb, w+, w+b). Unlinking and recreating the file is faster.