Send peers the IRC listener user's QUIT reason (closes #216)

handleQuit now records the reason on the connection, and cleanup
passes it to BroadcastQuit, the same service call the HTTP API's
QUIT uses. A QUIT without a reason still gives Client quit. A
connection that ends without a QUIT still gives Connection closed.

The relay test that expected Connection closed for a bare QUIT now
expects Client quit.

Model: opus-5-5

internal/ircserver/commands.go

@@ -371,6 +371,7 @@ func (c *Conn) handleQuit(msg *Message) {
 
 	c.mu.Lock()
 	c.closed = true
+	c.quitReason = reason
 	c.mu.Unlock()
 }