Comment boot block checks and perform additional sanity checks:

Comment boot block checks and perform additional sanity checks:

The following checks are now being enforced:

 - bpbBytesPerSec: only accept 512, 1024, 2048 and 4096.
 - bpbSecPerClust: only accept 1, 2, 4, 8, 16, 32, 64 and 128.
 - bpbResSectors: require non-zero.
 - bpbFATs: require non-zero.
 - bpbSectors: require zero for FAT32.
 - bpbFATsmall: require zero for FAT32.
 - bpbHugeSectors: require non-zero for FAT32.

Bail out if the BPB contained values that do not meet these requirements.

We also require FATsecs * FATsecs to not overflow 32-bit unsigned
integer.

Check for backup boot block was removed because the checker does not take
corrective action, and msdosfs driver ignores it too.