opt: don't disable SO_REUSEPORT on DragonFlyBSD

Fixes #582

gnet.go

@@ -476,8 +476,11 @@ func createListeners(addrs []string, opts ...Option) ([]*listener, *Options, err
 	//
 	// Note that FreeBSD 12 introduced a new socket option named SO_REUSEPORT_LB
 	// with the capability of load balancing, it's the equivalent of Linux's SO_REUSEPORT.
+	// Also note that DragonFlyBSD 3.6.0 extended SO_REUSEPORT to distribute workload to
+	// available sockets, which make it the same as Linux's SO_REUSEPORT.
 	goos := runtime.GOOS
-	if (options.Multicore || options.NumEventLoop > 1) && options.ReusePort && goos != "linux" && goos != "freebsd" {
+	if (options.Multicore || options.NumEventLoop > 1) && options.ReusePort &&
+		goos != "linux" && goos != "dragonfly" && goos != "freebsd" {
 		options.ReusePort = false
 	}