Support for larger disks.

Support for larger disks.
- MFS, df(1), fsck(1), badblocks(8), de(1x) now compute the
  superblock's s_firstdatazone value if the on-disk value is zero
- mkfs(1) sets s_firstdatazone in the superblock to zero if the
  on-disk field is too small to store the actual value
- more agressive mkfs(1) inode number heuristic, copied from r5261