Re: kern/60456: FFS_NO_SNAPSHOT option is not entirely effective

Perhaps we should have an option that elides all of the file system
snapshot code, but adds a small check so that if you try to mount a
file system with any snapshots, it refuses to mount read/write -- only
allows mounting file systems with snapshots read-only, so that you
won't inadvertently corrupt the snapshots.

(That said: it's really nice to have fs-internal snapshots, for
reliable backups and for system updates, without having to rely on a
snapshot mechanism for the storage medium.  Maybe if you're just
running a VM, or you're booting from an SD card that you can easily
swap, none of that matters, but if the storage medium itself isn't
easily snapshottable or swappable, ffs snapshots are very useful.)