We try mounting UFS partitions first as ufs, then ufs2.  Make sure to
send stderr to /dev/null in case the mount fails in either case.  We
don't want the user to see that.

Also, don't try updating /etc/dumpdates or /etc/fstab unless they exist.
If we're fixing up a disk image, slicefix will do *all* slices including
those that don't have a root filesystem.