libbe(3): allow creation of arbitrary depth boot environments

libbe currently only provides an API to create a recursive boot environment,
without any formal support for intentionally limiting the depth. This
changeset adds an API, be_create_depth, that may be used to arbitrarily
restrict the depth of the new BE.

Submitted by: Rob Fairbanks <rob.fx907 gmail com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18564