More robust ibm autodetection for 77-track formats

[tdaede]

Currently, garbage on tracks 77-81 can confuse ibm track autodetection, resulting in writing a sparse .img.

In particular, I have some 77-track floppies that were factory formatted for 80 tracks @ 512 byte sectors, but formatted by the machine with 77 tracks @ 1024 byte sectors.

This can be solved by manually specifying the format, but a heuristic could probably properly discard the remaining tracks, at least for .img format.