Support 81 track d81 images

[markusC64]

Well, Geos Wheels 4.1, 4.2 and 4.4 (both C64 & C128) has copy protection on track 81 for master disks and therefore need this extended format.

[mrdudz]

To make this future proof, it should perhaps support 83 tracks though? Not that any real software actually uses it (and most drives can only step to track 82) - but the physical maximum of (some) drives is 83, not 81 :)