cluster: support stdio option for workers

This commit allows setupMaster() to configure the stdio channels
for worker processes.

Refs: https://github.com/nodejs/node-v0.x-archive/issues/5727
Refs: https://github.com/nodejs/node/pull/7811
PR-URL: https://github.com/nodejs/node/pull/7838


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>