install: Slightly tighten up CLI parsing.

This causes us to give an error if you pass the installer any
positional arguments, e.g. with `--`.  There's no reason you'd want
to do this, but I accidentally did it by passing an extra `--` to
the `test-install/install` wrapper and spent a few minutes on
confused debugging.

scripts/lib/install

@@ -43,11 +43,16 @@ while true; do
             shift
             ;;
         --)
+            shift
             break
             ;;
     esac
 done
 
+if [ "$#" -gt 0 ]; then
+    usage
+fi
+
 if [ -n "$show_help" ]; then
     usage
 fi