Commands.txt: Streamline references to IRC+ protocol
Commands.txt: Update more descriptions

Update descriptions of "AWAY", "JOIN", "LIST", "MODE", "PART",
"TOPC", and "TRACE".
Update irc-channel.c to use irc-macros.h

This includes code cleanups, too :-)
Update irc-mode.c to use irc-macros.h

This includes code cleanups and more comments, too.