pk3 files created on Windows are corrupted on Linux

[Aciz]

The packing algorithm uses backslashes as path separators on Windows, which results in corrupted archives on Linux. Files packed on Linux can be opened fine on Windows.

This behavior is standard when using some Windows tools to create zip files (pwsh Compress-Archive for example), although it does in fact break the ZIP file specification (section 4.4.17.1)

   4.4.17 file name: (Variable)

       4.4.17.1 The name of the file, with optional relative path.
       The path stored MUST NOT contain a drive or
       device letter, or a leading slash.  All slashes
       MUST be forward slashes '/' as opposed to
       backwards slashes '\' for compatibility with Amiga
       and UNIX file systems etc.  If input came from standard
       input, there is no file name field.