Do not cache stdout or stderr when there is no callback

[renorzr]

Sometimes we need to stream output of exec, there could be large amount of data. We use "event" instead of "callback" on this situation, but still limited by cache of stdout.

[bnoordhuis]

Can you explain the purpose of this PR to me? The callback is not optional according to the documentation and though we don't enforce that right now, that's how you should treat it. Not passing in a callback is a bug in your application.

Is the idea to create a version of child_process.spawn() that runs your command in a shell?

[Nodejs-Jenkins]

Can one of the admins verify this patch?

[renorzr]

@bnoordhuis is right, this is not needed since there is already child_process.spawn. closed.