ircd-side WHY

[edk0]

Ideally as part of a larger rework of (in particular ext-) bans and how they're explained—in an ideal world, trying to speak in a $~a channel would tell you need to register. I don't think we need to solve SAT for this, just keep track of whether truth at this node of an imaginary expression tree makes the expression structurally trivial. If it does we've found a usable reason.