Respond with the (RFC3977) 480 response when authentication is required

instead of the RFC977 502. Reportedly, this will trigger Thunderbird to prompt
for or send client credentials. This should fix issue #208.

exec/nntpservice.js

@@ -267,7 +267,7 @@ while(client.socket.is_connected && !quit) {
 	}
 
 	if(!logged_in) {
-		writeln("502 Authentication required");
+		writeln("480 Authentication required");
 		log(LOG_WARNING,"!Authentication required");
 		continue;
 	}