child_process: don't `resume()` created socket

Calling `resume()` on a stream switches it to the old mode which causes
piping stdio from a child process to fail.

Fixes joyent/node#4510.
stream: Clean up more effectively in pipe()
child_process: Pull through untouched stdio streams

Otherwise child procs will never emit a 'close' event if you don't
ever consume their streams, because they will never hit the EOF.