Fix '-b' option to work with the superblock locations output by newfs

Fix '-b' option to work with the superblock locations output by newfs
on 4096-byte sector disks. No-op on 512-byte sector disks.

Adhering to the bedrock idea that ffs will *never* use anything but
512-byte block disk addressing (a.k.a. daddr_t), and believing that
the 'fake' bread()/bwrite() functions should always be using 512-btye
block numbers like their kernel big brothers, nuke the computed
dev_bsize and use DEV_BSIZE for conversions to/from byte offsets.

Spotted and various fixes tested by David Vasek.

Still to fix: scanning for superblocks on 4K disks.

ok otto@