A few places created strings based on the terminal width, using fixed-sized
buffers and without checking for overflowing them. Fix those, and also
replace all other sprintf() calls with snprintf().
Reported by cpet.
Chatnet doesn't use the odd 310 numeric support anymore, it now uses 310 for
the same WHOIS_HELPFUL as Dalnet. They apparently use a pretty bog-standard
Unreal ircd now.
