Always set the magic sequence when we write, rather than trusting the
previously read boot code to do so.
Use explicit encoding/decoding of little endian disk image.
Remove a comment which was OBE.
Change the test vector for "fdisk -I" to reflect that there is a magic
sequence in the result now.
Add test case for "fdisk" which reads the image back.
At least for the two test-cases this program now gives the same result
on sparc64 as on i386. The lack of an installed /boot/mbr on sparc64
raises an (un)interesting question.
