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
13c62c50e3