When a user has set mode "b", all private messages and notices to this
user are blocked if they don't originate from a registered user, an IRC Op,
server or service. The originator gets an error numeric sent back in this
case, ERR_CANNOTSENDTONICK_MSG(976), which is/was(?) used by KineIRCd, too.
This closes bug #144.
9d97004a28