child_process: add public API for IPC channel

This commit adds a public channel property to ChildProcess. The
existing _channel property is aliased to the new property, with
the intention of deprecating and removing it in the future.

Fixes: https://github.com/nodejs/node/issues/9313
PR-URL: https://github.com/nodejs/node/pull/9322


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>