unix,stream: fix getting the correct fd for a handle

On OSX it's possible that the fd is replaced, so use the proper libuv
API to get the correct fd.

PR-URL: https://github.com/nodejs/node/pull/6753


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>