change functions API overhaul:
- rename ZIP_CMD_INIT to ZIP_CMD_OPEN
- ZIP_CMD_CLOSE is called when reading is done
- add ZIP_CMD_FREE to free resources
- ZIP_CMD_STAT is called to retrieve meta info (time stamp, compression
        method, sizes, crc)
- add ZIP_CMD_ERROR to return error codes

get rid of (now unused) zip_entry members ch_flags and ch_mtime

--HG--
branch : HEAD