cmd/tailscaled, logtail: share link monitor from wgengine to logtail

Part of overall effort to clean up, unify, use link monitoring more,
and make Tailscale quieter when all networks are down. This is especially
bad on macOS where we can get killed for not being polite it seems.
(But we should be polite in any case)

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

cmd/tailscaled/tailscaled.go

@@ -36,6 +36,7 @@ import (
 	"tailscale.com/version"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/magicsock"
+	"tailscale.com/wgengine/monitor"
 	"tailscale.com/wgengine/netstack"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/tstun"
@@ -195,6 +196,12 @@ func run() error {
 		go runDebugServer(debugMux, args.debug)
 	}
 
+	linkMon, err := monitor.New(logf)
+	if err != nil {
+		log.Fatalf("creating link monitor: %v", err)
+	}
+	pol.Logtail.SetLinkMonitor(linkMon)
+
 	var socksListener net.Listener
 	if args.socksAddr != "" {
 		var err error
@@ -205,7 +212,8 @@ func run() error {
 	}
 
 	conf := wgengine.Config{
-		ListenPort: args.port,
+		ListenPort:  args.port,
+		LinkMonitor: linkMon,
 	}
 	if args.tunname == "userspace-networking" {
 		conf.TUN = tstun.NewFakeTUN()