libbe(3): Fix BE activation promoting activated BE

This allows older BEs to be destroyed as they become replaced by a BE
created from them: e.g.

bectl create -e brokenworld fixedworld
bectl activate fixedworld
bectl destroy brokenworld

Submitted by: Shawn Webb
Approved by: re (gjb)
Obtained from: HardenedBSD (5948c0581e)