net/dns{,/resolver}: refactor DNS forwarder, send out of right link on macOS/iOS

Fixes #2224
Fixes tailscale/corp#2045

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/macios_ext.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin,ts_macext ios,ts_macext
+
+package resolver
+
+import (
+	"errors"
+	"net"
+
+	"tailscale.com/net/netns"
+	"tailscale.com/wgengine/monitor"
+)
+
+func init() {
+	initListenConfig = initListenConfigNetworkExtension
+}
+
+func initListenConfigNetworkExtension(nc *net.ListenConfig, mon *monitor.Mon, tunName string) error {
+	nif, ok := mon.InterfaceState().Interface[tunName]
+	if !ok {
+		return errors.New("utun not found")
+	}
+	return netns.SetListenConfigInterfaceIndex(nc, nif.Interface.Index)
+}