A number of improvements to ZIP support.

• Handles entries with compressed size >2GB (signed/unsigned cleanup)
• Handles entries with compressed size >4GB ("ZIP64" extension)
• Handles Unix extensions (ctime, atime, mtime, mode, uid, etc)
• Format-specific "skip data" override allows ZIP reader to skip entries without decompressing them, which makes "tar -t" a lot faster.
• Handles "length-at-end" entries generated by, e.g., "zip -r - foo"

Many thanks to: Dan Nelson, who contributed the code and test files for

the first three items above and suggested the fourth.