A major overhaul of I/O routines, creating perform_io().

A major overhaul of I/O routines, creating perform_io().

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).