The machine booted properly (into X via xdm)
   and I did an su, a few "sync"s, then "kill -TERM 1".  When I got
   the single-user shell, I "umount -av", then "fsck /dev/rsd0a".  It
   found a few problems (sorry, I didn't write them down), and I said "y"
   to all questions.  Immediately after it finished, I typed "reboot".

That's wrong.  The `umount -av' is not going to downgrade the root
file system to read-only.  If fsck(8) changed anything, those changes
may get blown away when you type `reboot'.

You need to at least type `mount -u -r /' to make sure / is mounted
read-only.