Miscellaneous improvements to "jar".

[Ulf Zibis]

> Find suitable abstractions and refactor them into a separate piece of 
> code.
>
> The win is a lot bigger if you make the new abstractions
> public supported parts of the API,
> but that is harder with the JDK.

Yep, I agree. Often that's my motivation to report RFE's e.g.:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6798511
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6695386
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6595143
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6812862
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6826329

-Ulf