PK3: Proper ignorance for file comments/extra data

Fixes a couple issues:

• Extrainfo and file comments were not skipped for each file while parsing the central directory, meaning the game would report it as corrupt if any of those were non-zero. This can be the case for files made using the 7Zip CLI on Windows, which store an NTFS header per entry.
• The length of extrainfo was being added to the file contents offset for some reason, so after fixing the above issue, none of the lump data decompressed properly.

I haven't tested this fully, but this does allow PK3s generated using the 7Zip CLI to load into the game, although they may not function 100%. I doubt this will negatively impact SLADE-generated PK3s, since they presumably write nothing for any of those fields. There's still the issue with assumed entry order to deal with for full functionality, I think?