cmd/geth: require valid command as argument (or no arg)

cmd/geth/main.go

@@ -251,6 +251,9 @@ func main() {
 // It creates a default node based on the command line arguments and runs it in
 // blocking mode, waiting for it to be shut down.
 func geth(ctx *cli.Context) error {
+	if args := ctx.Args(); len(args) > 0 {
+		return fmt.Errorf("invalid command: %q", args[0])
+	}
 	node := makeFullNode(ctx)
 	startNode(ctx, node)
 	node.Wait()