Prevent downstreams from changing their nick to service's

This commit prevents downstream from sending those commands:
- NICK BouncerServ
- NICK BouncerServ/<network>

The later is necessary because soju would otherwise save the nick change
and, in the event that the downstream connects in single-upstream mode
to <network>, it will end up with the nickname "BouncerServ".

downstream.go

@@ -1068,11 +1068,12 @@ func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
 			Params:  []string{dc.nick, "You may not reregister"},
 		}}
 	case "NICK":
-		var nick string
-		if err := parseMessageParams(msg, &nick); err != nil {
+		var rawNick string
+		if err := parseMessageParams(msg, &rawNick); err != nil {
 			return err
 		}
 
+		nick := rawNick
 		var upstream *upstreamConn
 		if dc.upstream() == nil {
 			uc, unmarshaledNick, err := dc.unmarshalEntity(nick)
@@ -1088,6 +1089,12 @@ func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
 				Params:  []string{dc.nick, nick, "contains illegal characters"},
 			}}
 		}
+		if nick == serviceNick {
+			return ircError{&irc.Message{
+				Command: irc.ERR_NICKNAMEINUSE,
+				Params:  []string{dc.nick, rawNick, "Nickname reserved for bouncer service"},
+			}}
+		}
 
 		var err error
 		dc.forEachNetwork(func(n *network) {