when sending ObscureKeystrokeTiming chaff packets, we can't

when sending ObscureKeystrokeTiming chaff packets, we can't
rely on channel_did_enqueue to tell that there is data to send.
This flag indicates that the channels code enqueued a packet
on _this_ ppoll() iteration, not that data was enqueued in _any_
ppoll() iteration in the timeslice. ok markus@