[DALnet-services] add a channel mode to block part/quit message

hello everyone, i remember i'd seen something in UnrealIRCd long time ago, that permits to block part/quit message definitly for each channel, or with a delay. i wonder if it will be possible to add this to DALnet. maybe like a channel mode. it could help ops to fight against some spam/flood. and do be honnest, i actually need it :) Ark.