bectl(8): destroy: use BE_DESTROY_AUTOORIGIN if -o is not specified

-o will force the origin to be destroyed unconditionally.
BE_DESTROY_AUTOORIGIN, on the other hand, will only destroy the origin if it
matches the format used by be_snapshot. This lets us clean up the snapshots
that are clearly not user-managed (because we're creating them) while
leaving user-created snapshots in place and warning that they're still
around when the BE created goes away.