Improved extraction of encrypted archives

Extraction of ecrypted (i.e. password protected) has been further improved. We now check if a rar-file is encrypted before we try to extract. This saves a lot of time and resources (especially when using large multi-part archives):

- When extraction of archives that are not password protected (but contain CRC errors) fails, unrarall will no longer try to extract them using every single password in the password list.

- When an archive really needs a password, unrarall will not try to extract it without a password first.