Add NotifyStartedFunc field to Server

Adds a field, NotifyStartedFunc func() to Server.

If non-nil, it is called after a server starts listening. This is useful
for synchronization purposes, for example when a daemon needs to drop
privileges after binding. Otherwise, there is no way to determine when
the server has begun listening and hardcoded delays (!) must be used or
race conditions may occur.

server.go

@@ -223,6 +223,9 @@ type Server struct {
 	// Unsafe instructs the server to disregard any sanity checks and directly hand the message to
 	// the handler. It will specfically not check if the query has the QR bit not set.
 	Unsafe bool
+	// If set, called once the server has started listening. This is useful if
+	// you need to drop privileges only after binding.
+	NotifyStartedFunc func()
 
 	// For graceful shutdown.
 	stopUDP chan bool
@@ -370,6 +373,11 @@ func (srv *Server) getReadTimeout() time.Duration {
 // Each request is handled in a seperate goroutine.
 func (srv *Server) serveTCP(l *net.TCPListener) error {
 	defer l.Close()
+
+	if srv.NotifyStartedFunc != nil {
+		srv.NotifyStartedFunc()
+	}
+
 	handler := srv.Handler
 	if handler == nil {
 		handler = DefaultServeMux
@@ -401,6 +409,10 @@ func (srv *Server) serveTCP(l *net.TCPListener) error {
 func (srv *Server) serveUDP(l *net.UDPConn) error {
 	defer l.Close()
 
+	if srv.NotifyStartedFunc != nil {
+		srv.NotifyStartedFunc()
+	}
+
 	handler := srv.Handler
 	if handler == nil {
 		handler = DefaultServeMux