child_process: expose ChildProcess constructor

Creates two new internal modules (child_process and socket_list) for
better readability.

Exposes the ChildProcess constructor from the child_process module so
one can now `require(‘child_process’).ChildProcess`

Fixes: https://github.com/nodejs/io.js/issues/1751
PR-URL: https://github.com/nodejs/io.js/pull/1760


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>