[FASTFAT] Completely rewrite support for dirty volumes.

[FASTFAT] Completely rewrite support for dirty volumes.

Until now, our support for dirty volumes was totally broken
to a point where, on FAT32 volume, the dirty couldn't even
be written nor read from the disk.

This commit totally rewrites its handling, for both FAT16 and FAT32
so that it's now fully functionnal. Furthermore, it also gets
totally compatible with our vfatlib, and thus, autochk.
Now, on mount, FastFAT will check if the volume is dirty or not, and
autochk will be able to ask for a repair if dirty. vfatlib will
repair the volume and remove the dirty bit. So that, on next
reboot, the volume will be mounted clean.

As a reminder, the dirty bit is set immediately after mounting
the volume, so that, if you crash or have a powercut, autochk
will always attempt to repair your volume (with more or less,
that's FAT!).

If you want to experience without breaking your FAT volume,
just boot, open a cmd prompt and type: fsutil dirty set c:
and reboot!

CORE-13758
CORE-13760
CORE-13759