formats/dmk_dsk.cpp: Improve checks in identify.

[wilbertpol]

And be more lenient towards images where the amount of stored tracks does not match the amount of tracks set in the header.

[ajrhacker]

I've thought about this subject since the discussion on #12062, and I've come to the conclusion that performing such "structural" checks on stored data is simply not warranted, especially due to the high priority of FIFID_STRUCT. It's way too common for fields such as volume names to disagree with official or unofficial specifications.

[wilbertpol]

I guess that depends on how you interpret structure: the structure of the file format or the structure of the actual data stored in the file.
If FIFID_STRUCT a too high a priority for the simple file format checks done here then it'd be better to use FIFID_HINT?

[ajrhacker]

@galibert commented on #12062 that "Pure sector formats can't have a structural or a signature check in any case." I blame him for not making that clear when he added FIFID_STRUCT.

[wilbertpol]

Right, then both FIFID_STRUCT and FIFID_SIGN are too strong for this format. I changed it to use FIFID_HINT.