bectl(8): Add -o flag to destroy to clean up the origin snapshot of BE

We can't predict when destruction of origin is needed, and currently we have
a precedent for not prompting for things. Leave the decision up to the user
of bectl(8) if they want the origin snapshot to be destroyed or not.

Emits a warning when -o isn't used and an origin snapshot is left to be
cleaned up, for the time being. This is handy when one drops the -o flag but
really did want to clean up the origin.

A couple of -e ignore's have been sprinkled around the test suite for places
that we don't care that the origin's not been cleaned up. -o functionality
tests will be added in the future, but are omitted for now to reduce
conflicts with work in flight to fix bits of the tests.

Reported by: Shawn Webb
MFC after: 1 week