ipn: add a WatchIPNBus option bit to subscribe to EngineStatus changes

So GUI clients don't need to poll for it.

We still poll internally (for now!) but that's still cheaper. And will
get much cheaper later, without having to modify clients once they
start sending this bit.

Change-Id: I36647b701c8d1fe197677e5eb76f6894e8ff79f7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/backend.go

@@ -52,6 +52,17 @@ type EngineStatus struct {
 	LivePeers      map[key.NodePublic]ipnstate.PeerStatusLite
 }
 
+// NotifyWatchOpt is a bitmask of options about what type of Notify messages
+// to subscribe to.
+type NotifyWatchOpt uint64
+
+const (
+	// NotifyWatchEngineUpdates, if set, causes Engine updates to be sent to the
+	// client either regularly or when they change, without having to ask for
+	// each one via RequestEngineStatus.
+	NotifyWatchEngineUpdates NotifyWatchOpt = 1 << iota
+)
+
 // Notify is a communication from a backend (e.g. tailscaled) to a frontend
 // (cmd/tailscale, iOS, macOS, Win Tasktray).
 // In any given notification, any or all of these may be nil, meaning