Make sure a write error drains any messages in the input buffer.

(while keeping it backward compatible). Improve --bwlimit docs.

- The receiver notifies the generator if it is exiting with an error,
  and then, if it is a server, waits around for the generator to die.
  This ensures that the client side has time to read the error.
- The generator or sender will notifiy the other side of the transfer of
  an error-exit value if protocol 31 is in effect.  This will get rid of
  some "connection unexpectedly closed" errors that are really expected
  events due to a fatal exit on the other side.

in an incremental recursion scan.

that span two reads.

so that the client will be able to cooperate with keep-alive.