tailcfg, control/controlclient, ipn/ipnlocal: add c2n (control-to-node) system

This lets the control plane can make HTTP requests to nodes.

Then we can use this for future things rather than slapping more stuff
into MapResponse, etc.

Change-Id: Ic802078c50d33653ae1f79d1e5257e7ade4408fd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1075,6 +1075,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 		PopBrowserURL:        b.tellClientToBrowseToURL,
 		Dialer:               b.Dialer(),
 		Status:               b.setClientStatus,
+		C2NHandler:           http.HandlerFunc(b.handleC2N),
 
 		// Don't warn about broken Linux IP forwarding when
 		// netstack is being used.