ipn/ipnlocal, control/controlclient: propagate link monitor to controlclient

Don't use it yet, but get it down there.

Updates #1455

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

ipn/ipnlocal/local.go

@@ -554,6 +554,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 		HTTPTestClient:    opts.HTTPTestClient,
 		DiscoPublicKey:    discoPublic,
 		DebugFlags:        controlDebugFlags,
+		LinkMonitor:       b.e.GetLinkMonitor(),
 	})
 	if err != nil {
 		return err