Support ISOs with a non-standard PVD layouts (#2238)

Support ISOs with a non-standard PVD layouts (#2238)

The CSRG ISOs have a non-standard PVD layout with a 68-byte root
directory record (rather than the 34-byte record required by
ECMA119/ISO9660). I built a test image with this change and modified the
ISO9660 reader to accept it.

While I was working on the bid logic to recognize PVDs, I added a number
of additional correctness checks that should make our bidding a bit more
accurate. In particular, this should more than compensate for the
weakened check of the root directory record size.

Resolves #2232