Fix encoding problems with ZipFile

SharpZipLib seems to use default "OEM" encoding for decoding the file names in the archive. This may cause problems if the default OEM encoding (as reported by mono) do not installed in the system.

Mono seems to select the OEM code page based on system locale. On Russian system it is 866, on Greek - 737 (as reported by #5261). On some system these "OEM" code pages are not installed and the game crashes.

I think it's sufficient to use the default encoding for file names (as we hopefully don't store some non-ASCII filenames there).

This patch should fix bug #5261.