child_process: support stdio option in fork()

This commit allows child_process.fork() to pass stdio options
to spawn(). This allows fork() to more easily take advantage of
additional stdio channels.

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


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>