health,ipn/ipnlocal: introduce eventbus in heath.Tracker (#17085)

The Tracker was using direct callbacks to ipnlocal. This PR moves those
to be triggered via the eventbus.

Additionally, the eventbus is now closed on exit from tailscaled
explicitly, and health is now a SubSystem in tsd.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

cmd/tailscaled/debug.go

@@ -161,7 +161,9 @@ func getURL(ctx context.Context, urlStr string) error {
 }
 
 func checkDerp(ctx context.Context, derpRegion string) (err error) {
-	ht := new(health.Tracker)
+	bus := eventbus.New()
+	defer bus.Close()
+	ht := health.NewTracker(bus)
 	req, err := http.NewRequestWithContext(ctx, "GET", ipn.DefaultControlURL+"/derpmap/default", nil)
 	if err != nil {
 		return fmt.Errorf("create derp map request: %w", err)