bectl(8): create non-recursive boot environments

bectl advertises that it has the ability to create recursive and
non-recursive boot environments. This patch implements that functionality
using the be_create_depth API provided by libbe. With this patch, bectl now
works as bectl(8) describes in regards to creating recursive/non-recursive
boot environments.

Submitted by: Rob Fairbanks <rob.fx907 gmail com> (with minor changes)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20240