Import libdsk-1.3.3 as wip/libdsk.

LibDsk is a library intended to give transparent access to floppy
drives and to the "disc image files" used by emulators to represent
floppy drives. It currently supports the following disc image
formats:

- Raw "dd if=foo of=bar" images;
- Raw images in logical filesystem order;
- CPCEMU-format .DSK images (normal and extended);
- CFI-format disc images, as produced by FDCOPY.COM;
- ApriDisk-format disc images;
- NanoWasp-format disc images, used by the eponymous emulator;
- Yaze 'ydsk' disc images, created by the 'yaze' emulators;
- Disc images created by Teledisk and CopyQM (read only);
- The floppy drive under Linux.
Add libdsk.