formats/imd_dsk: implement get_sectors_per_track()

[lkundrak]

Otherwise imgtool gets unhappy with IMD images:

Starting program: ./imgtool identify ./altos586/SDX-586.IMD
...
Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x000000000025d426 in bml3_diskimage_open (image=..., dummy=...)
at .../src/tools/imgtool/modules/bml3.cpp:520
#2 0x000000000024807d in imgtool_floppy_open_internal (stream=..., image=...)
at .../src/tools/imgtool/iflopimg.cpp:83
#3 imgtool_floppy_open (image=..., stream=...)
at .../src/tools/imgtool/iflopimg.cpp:101
#4 0x000000000024bbdf in imgtool::image::internal_open
(module=module@entry=0x302320, filename="./altos586/SDX-586.IMD",
read_or_write=read_or_write@entry=0, createopts=createopts@entry=0x0,
outimg=std::unique_ptrimgtool::image = {...})
at .../src/tools/imgtool/imgtool.cpp:986
#5 0x000000000024bd19 in imgtool::image::open (module=module@entry=0x302320,
filename="./altos586/SDX-586.IMD", read_or_write=read_or_write@entry=0,
outimg=std::unique_ptrimgtool::image = {...})
at .../src/tools/imgtool/imgtool.cpp:1010
#6 0x0000000000250413 in evaluate_module (fname=fname@entry=0x2fa610
"./altos586/SDX-586.IMD", module=0x302320, result=@0x7fffffffd4dc: 0)
at .../src/tools/imgtool/imgtool.cpp:271
#7 0x000000000025075f in imgtool::image::identify_file (fname=0x2fa610
"./altos586/SDX-586.IMD", modules=modules@entry=0x7fffffffd5d0, count=127,
count@entry=128)
at/usr/include/c++/13/bits/unique_ptr.h:199
#8 0x000000000025382e in cmd_identify (c=0x229d50 <cmds+400>, argc=, argv=0x7fffffffda00)
at .../src/tools/imgtool/main.cpp:578
#9 0x0000000000253efc in main (argc=3, argv=0x7fffffffd9f0)
at .../src/tools/imgtool/main.cpp:939
(gdb) up
#1 0x000000000025d426 in bml3_diskimage_open (image=..., dummy=...)
at .../src/tools/imgtool/modules/bml3.cpp:520
520 int sectors_per_track = callbacks->get_sectors_per_track(floppy, 0, 20);
(gdb) print *callbacks
$2 = {read_sector = 0x28de1c <imd_read_sector(...)>,
write_sector = 0x0,
read_indexed_sector = 0x28de03 <imd_read_indexed_sector(...)>,
write_indexed_sector = 0x28de35 <imd_write_indexed_sector(...)>,
read_track = 0x0,
write_track = 0x0,
format_track = 0x0,
post_format = 0x0,
get_heads_per_disk = 0x28dfef <imd_get_heads_per_disk(...)>,
get_tracks_per_disk = 0x28dfff <imd_get_tracks_per_disk(...)>,
get_sectors_per_track = 0x0,
get_track_size = 0x0,
get_sector_length = 0x28dd1c <imd_get_sector_length(...)>,
get_indexed_sector_info = 0x28d95c <imd_get_indexed_sector_info(...)>,
get_track_data_offset = 0x0}
(gdb)

With patch applied:

$ ./imgtool identify ./altos586/SDX-586.IMD

imd_vzdos IMD floppy disk image (VZ-DOS format)
imd_dgndos IMD floppy disk image (Dragon DOS format)