libblkid: fix MBR detection on iPod and cleanup vfat code

libblkid: fix MBR detection on iPod and cleanup vfat code

 - move all FAT code to superblocks/vfat.c only
 - add a generic function to verify FAT superblock and use it
   in FAT prober as well as in MBR parser
 - add a more robust FAT cluster_count check
   (it seems that iPod contains an "almost valid" FAT superblock before MBR)

Reported-by: Davidlohr Bueso <dave.bueso@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>