MSG_NOOP is not available (is both safer and supports older rsyncs).

and no one is reading (i.e. do advantageous reading in perform_io()).

Added some debug output for MSG_ERROR_EXIT messages.

(Patch from Matt.)

all the time (we only waste from 1-3 bytes some of the time).

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

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