zip_get_num_files - get number of files in archive
       libzip (-lzip)
       #include <zip.h>
       int zip_get_num_files(struct zip *archive);
       The  zip_get_num_files function returns the number of files in archive.
       zip_get_num_files returns the number of files in the zip archive, or -1
       if archive is NULL.
       libzip(3)
       Dieter Baron <dillo@giga.or.at> and Thomas Klausner <tk@giga.or.at>