print out the values of the bytes that do not match.
Add comment explaining that there is no documented rationale for the
check.
fsck_msdos checks that several bytes are equal in the primary and
backup bootblock. There is no documented rationale. The kernel does
not enforce this when mounting a filesystem. I have a FAT32
filesystem on my disk (mounted as /share) which fails this check, but
is accepted by both Windows XP and NetBSD. My volume differs in bytes
71-81. The primary contains "SHARE " and the backup NUL bytes.
Further, fsck_msdos does not have code to repair the mismatch. As a
result such filesystems can be used not checked.
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