Setting to (not) get back own messages reliably

[RAnders00]

Three states:

• "Doesn't matter" - when a PRIVMSG message is sent, the connection chosen doesn't matter.
• "Must not be joined" - only send PRIVMSG messages to connections where that channel isn't joined. This means the library user gets their own messages back as PRIVMSG, reliably. If the channel is not joined on any of the connections then the selected connection doesn't matter.
• "Must be joined" - only send PRIVMSG messages to the connection that is joined to that channel. If the channel is not joined on any of the connections then the selected connection doesn't matter. This means potentially sending messages even if the connection would otherwise currently be too busy.