child_process: add keepOpen option to send()

This option allows an instance of net.Socket to be kept open in
the sending process.

Fixes: https://github.com/nodejs/node/issues/4271
PR-URL: https://github.com/nodejs/node/pull/5283


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