Miscellaneous improvements to "jar".

[Ulf Zibis]

Additional 2 cents, I more would like:
>
> +        return File.createTempFile("jartmp", null, (dir != null ? dir 
> : new File(".")));

or:

+        return File.createTempFile("jartmp", null,
+               dir != null ? dir : new File("."));

-Ulf