Just for reference, I've decided that mount is too lax about
dirty flags; on a change to multi-user, the system just complains that
all the filesystems are dirty - but still mounts them!  Since you can't umount
/usr ("device busy"), you then have to hard boot immediately to recover
usefully, ... see, copying a kernel to /dev/reload syncs disks.

Maybe a /dev/rreload which doesn't sync would help...  I'd just like
to see fsck be more agressive.

-s