cd9660 pointer sign issues and missing __packed attribute

The isonum_* functions are defined to take unsigend char* as an argument,
but the structure fields are defined as char. Change to u_char where needed.

Probably the full structure should be changed, but I'm not sure about the
side affects.

While there, add __packed attribute.

Differential Revision: https://reviews.freebsd.org/D16564