irc-cap.c: move static functions to the top of the file

And remove now unnecessary (local) prototypes.
Add new _IRC_ARGC_BETWEEN_OR_RETURN_ macro to irc-macros.h
Add new _IRC_ARGC_EQ_OR_RETURN_ macro to irc-macros.h
irc-cap.c: Update source code documentation
irc-op.c: Update source code documentation
irc-cap.c: use irc-macros.h
irc-op.c: use irc-macros.h
Commands.txt: Update description of the "INVITE" command
Commands.txt: Update description of the "KICK" command