Use symbolic constants from <sys/diskmbr.h> instead of local constants.

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.