ipn/ipnlocal: close foreground sessions on SetServeConfig

This PR ensures zombie foregrounds are shutdown if a new
ServeConfig is created that wipes the ongoing foreground ones.
For example, "tailscale serve|funnel reset|off" should close
all open sessions.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -752,9 +752,9 @@ func TestWatchNotificationsCallbacks(t *testing.T) {
 		}
 		// Send a notification. Range over notifyWatchers to get the channel
 		// because WatchNotifications doesn't expose the handle for it.
-		for _, c := range b.notifyWatchers {
+		for _, sess := range b.notifyWatchers {
 			select {
-			case c <- n:
+			case sess.ch <- n:
 			default:
 				t.Fatalf("could not send notification")
 			}