libbe: handle destroying/renaming temporary/bootonce boot environments

When a temporary/bootonce boot environment is renamed, we need to also
update the bootenv nvlist on-disk to reflect the new name. Additionally,
when a temporary/bootonce boot environment is destroyed, we also need to
clear out the on-disk state.

Reviewed by: kevans
Approved by: kp
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43591