Don't listen on all interfaces

This is the dumb fix for the face-palm bug reported here:

  #9

wherein our attempt to listening on IPv4 and IPv6 loopback interfaces
when available was actually causing us to listen on all interfaces
(ugh).

I'll remove the duplication in the next commit, and also try to
gracefully handle systems which might not have IPv6 interfaces up.

clipper.go

@@ -232,39 +232,61 @@ func main() {
 	}
 
 	var addr string
-	var listenType string
+	var listeners []net.Listener
 	if isPath(settings.Address) {
 		addr = expandPath(settings.Address)
 	} else {
 		addr = settings.Address
 	}
 	if strings.HasPrefix(addr, "/") {
-		listenType = "unix"
 		log.Print("Starting UNIX domain socket server at ", addr)
+		listener, err := net.Listen("unix", addr)
+		if err != nil {
+			log.Fatal(err)
+		}
+		listeners = append(listeners, listener)
 	} else {
-		listenType = "tcp"
 		if addr == "" {
 			log.Print("Starting TCP server on loopback interface")
+			addr = fmt.Sprintf("%s:%d", "127.0.0.1", settings.Port)
+			listener, err := net.Listen("tcp4", addr)
+			if err != nil {
+				log.Fatal(err)
+			}
+			listeners = append(listeners, listener)
+			addr = fmt.Sprintf("%s:%d", "[::1]", settings.Port)
+			listener, err = net.Listen("tcp6", addr)
+			if err != nil {
+				log.Fatal(err)
+			}
+			listeners = append(listeners, listener)
 		} else {
 			log.Print("Starting TCP server on ", addr)
+			addr = fmt.Sprintf("%s:%d", settings.Address, settings.Port)
+			listener, err := net.Listen("tcp", addr)
+			if err != nil {
+				log.Fatal(err)
+			}
+			listeners = append(listeners, listener)
 		}
-		addr = fmt.Sprintf("%s:%d", settings.Address, settings.Port)
 	}
-	listener, err := net.Listen(listenType, addr)
-	if err != nil {
-		log.Fatal(err)
+	for i := range listeners {
+		defer listeners[i].Close()
 	}
-	defer listener.Close()
 
 	go func() {
-		for {
-			conn, err := listener.Accept()
-			if err != nil {
-				log.Print(err)
-				return
-			}
+		for i := range listeners {
+			go func(listener net.Listener) {
+				for {
+					conn, err := listener.Accept()
+					if err != nil {
+						log.Print(err)
+						return
+					}
 
-			go handleConnection(conn)
+					go handleConnection(conn)
+				}
+			}(listeners[i])
 		}
 	}()