Add a user mode that only lets the user speak to operators

[tjfontaine]

This user mode would only be set by servers (similar to how +R works) this theoretical +T mode would only let a user speak to an operator (+o or +P) and wouldn't let them join any new channels, and wouldn't let them speak in any channels they are currently in.

I'm not sure if I care if they are notified or not that they can't speak, though I'm kinda in favor of not telling them.

[df7cb]

What we'd need today is a user mode that only permits talking to NickServ, so they can either register (and verify on the web), or identify. Once that is done, they can proceed normally.

[dwfreed]

Could probably just use the capture stuff for this (see contrib/m_capture)