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/relay_test.go

@@ -316,7 +316,7 @@ func TestRelayNickDiffersOnlyInCase(t *testing.T) {
 		{"JOIN #case", ":Alice!Alice@* JOIN #case"},
 		{"NICK Alicia", ":Alice!Alice@* NICK Alicia"},
 		{"NICK Alice", ":Alicia!Alicia@* NICK Alice"}, // back for QUIT
-		{"QUIT", ":Alice!Alice@* QUIT :Quit: Connection closed"},
+		{"QUIT", ":Alice!Alice@* QUIT :Quit: Client quit"},
 	} {
 		capitalAlice.send(step.line)