Decide what to do with self messages

[progval]

When a client sends a message, other clients of the same session don't get it unless they have echo-message enabled.
I believe this is similar to ZNC's behavior when the znc.in/self-message capability is not negotiated.

It was implemented this way in Sable because it's easier, not as a design choice.

IMO, Ergo's behavior to unconditionally send it makes more sense.

[spb]

I can get behind the Ergo behaviour as long as common clients don't get too confused by it.