Support client-only tags and TAGMSG

[csmith]

I'd like to be able to send a TAGMSG to other clients connected to the same account to synchronise things like read and unread state.

So if clientA and clientB were connected to the same account with nickname "bob", clientA would send:

@+dmdirc.com/markread=#channel TAGMSG bob

And clientB would get it echoed back in the same way as PRIVMSG works now:

@+dmdirc.com/markread=#channel :bob!bob@bobbington.bob TAGMSG bob

This probably needs two things changing/confirming in DFBnc:

• Accept client-only tags from clients
• Relay TAGMSG messages in the same manner as PRIVMSGs

[csmith]

Servers MUST NOT deliver TAGMSG to clients that haven’t negotiated the message tags capability. 1

So probably need to deal with that as well when echoing it back to clients.