wgengine/userspace: add support to automatically enable/disable the tailscale

protocol in BIRD, when the node is a primary subnet router as determined
by control.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscaled/tailscaled_bird.go

@@ -0,0 +1,19 @@
+// 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.
+
+//go:build linux || darwin || freebsd || openbsd
+// +build linux darwin freebsd openbsd
+
+package main
+
+import (
+	"tailscale.com/chirp"
+	"tailscale.com/wgengine"
+)
+
+func init() {
+	createBIRDClient = func(ctlSocket string) (wgengine.BIRDClient, error) {
+		return chirp.New(ctlSocket)
+	}
+}