Pk3 fixes

Here is a summary:

1. Add checking of extra fields and comments on zip files because otherwise the parser would mess up if present.

2)Fix the scanning of directories for function W_CheckNumForFolderStartPK3 since it skipped files built with zip archives other than slade.

3)Fix reading errors with pk3 archivers built with zip archivers like winrar and unix zip command line tool. (this patches https://mb.srb2.org/showthread.php?p=804347#post804347)