(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.

Files-from data is now sent as multiplexed I/O so that it can mingle
with any messages (such as debug output).  Requires protocol 31.

Protocol 31 no longer disables output verbosity in a couple instances
that used to cause protocol issues.

Got rid of MSG_* messages that have implied raw data that follows after
them.  We instead send a negative index value as a part of the raw data
stream, which is guaranteed to be output together with the following
data.  This only affects the (in-progress) protocol 31 and the (self-
contained) communication stream from the receiver to the generator.

Added --debug=IO and improved --debug=FLIST.  Some --debug=IO output
requires --msgs2stderr to be used to see it (i.e. sending a message
about sending a message would send another message, ad infinitum).

by using inline functions for the 4-char <-> uint32 conversions.