libbe(3): promote dependent clones when destroying an environment

When removing a boot environment iterate over the dependents and process the
snapshots by grabbing any clones. Promote the clones we found and then
remove the target environment.

This fixes the ability to destroy a boot environment when it has been used
to spawn one or more other boot environments.

PR: 242592
Submitted by: Wes Maag <jwmaag gmail com> (with changes by myself)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22953