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.
