some Channel fixes for max packet size & blocking on zero window
some clean-ups and fixes to channels:
* when send() is blocked on a zero-width window, check that the channel is
still open. this was causing some lockups.
* set a lower bound to the "maximum packet size" we accept from the remote
host. if they tell us anything less than 1KB, assume they meant 1KB. (it's
not reasonable to fragment below that.)
* leave a little padding instead of cutting right up to the maximum packet
size: some space will be taken up by protocol overhead.
* turn off some of the debug log lines unless "ultra_debug" is on (nobody
cares about the feed info)
cba104ce3f