convert to using the _daddr_t types like newfs was...

Put the superblock in the correct possition for UFS2... There is a bug
in FFS that if we don't put it here (for UFS2), it will forcefully
relocate the superblock, and I believe cause data loss..

I have a fix for that, but w/ how many releases are broken, we won't be
able to switch to the better _FLOPPY (block 0) for this for a while..