Add {VM.closeStream}

* Something similar is necessary for correct termination detection.
* What happened is the main Oz Thread waits on the stream,
  and nobody is there to wake it up so the VM just shuts down.
  This is similar to async IO, so we use the _asyncIONodeCount counter.
  But if we never call getStream(), we wish not to wait for it,
  since of course we can not wait on it.
* Applications using {VM.getStream} should now use {VM.closeStream}.
* TODO: What about GC and ProtectedNode?