jar --verify operation mode checking mrjar validity

[Christian Stein]

Hi,

jar --create (and --update) perform type API validation checks when used in
combination with --release option. This detects invalid "version overlays"
at package time, where the API doesn't match a previous one.

Having a jar --verify mode that performs the same checks consuming an
already existing jar file would be useful as most "3rd-party packaging
tools" don't perform those checks.

A possible work-around could be to explode an existing jar and re-create it
using jar --create...

Cheers,
Christian