Additions to T utilities

[Chris Riesbeck <Riesbeck at YALE>]

Bob Nix's favorite function, MSG, has been extended to take

  (MSG ... (TO stream) ...)

to redirect output (a local effect to the end of the MSG only),
as well as (BIND ((*MSG_STREAM* ...)) ...) to affect a bunch of
messages if they didn't explicitly say where they were going.

Also added (ERRMSG -cmds-) => (MSG (TO error-output) -cmds- (DO (ERROR)))
and  (TTYMSG -cmnds-) => (MSG (TO standard-output) -cmds-)

MSG, ERRMSG and TTYMSG are macros that expand into

  (LET ((msg_out_stream *MSG_STREAM*)) -command_expansions-)

where output commands expand to msg_out_stream and TO expands to
(SET msg_out_stream ...).
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