libbe(3): Add a destroy option for removing the origin

Currently origin snapshots are left behind when a BE is destroyed, whether
it was an auto-created snapshot or explicitly specified via, for example,
bectl create -e be@mysnap ....

Removing it automatically could be argued as a POLA violation in some
circumstances, so provide a flag to be_destroy for it. An accompanying
option will be added to bectl(8) to utilize this.

Some minor style/consistency nits in the affected areas also addressed.

Reported by: Shawn Webb
MFC after: 1 week