all: fork tailscale Linux dns manager package

With modification to fit our use case.

internal/dns/direct_notlinux.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2022 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.
+
+//go:build !linux
+
+package dns
+
+func (m *directManager) runFileWatcher() {
+	// Not implemented on other platforms. Maybe it could resort to polling.
+}