Admit b_blkno means block number; a block is DEV_BSIZE (a.k.a.

Admit b_blkno means block number; a block is DEV_BSIZE (a.k.a.
512) bytes; ffs is inextricably tied to using b_blkno and disklabel
always uses sectorsize units.

Thus use DEV_BSIZE units for all fields describing ffs filesystems
and convert to/from sectors where required. This enables the creation
and use of ffs filesystems on non-512 byte sectorsize devices.

This diff allows i386 and sgi (the two test platforms) to find
disklabels that are not on a sectorsize boundary. Same change to
further archs coming.

This is a no-op on 512-byte sectorsize devices.

This work triggered by jsing@'s need to create ffs filesystems on
sgi cdroms so we can create cdrom install media for sgi.

sgi testing by jsing@

ok jsing@ pedro@ "looks sane" beck@ weingart@