Demo mode idea.

1. user demo mode (+d) is automatically assigned via I:line flag (or set
   manually, but who would? maybe some web interfaces?);
   users with usermode +d we'll call "demousers".
2. if channel mode (+D) is set, demouser will not be able to chat there (unless +o/+v)
3. if recipient has user mode (+D) set, demouser will not be able to talk to him/her

(new numeric: "Recipient does not wish to receive messages from you"
or similar for users; channels can utilize old "cannot speak").

Q: What's the point of this mode?
A: Set it on dialup users, free shells; let other users choose, if they
   wish to receive messages from such users (possibly spam).