bectl(3)/libbe(3): Allow BE root to be specified

Add an undocumented -r option preceding the bectl subcommand to specify a BE
root to operate out of. This will remain undocumented for now, as some
caveats apply:

• BEs cannot be activated in the pool that doesn't contain the rootfs
• bectl create cannot work out of the box without the -e option right now, since it defaults to the rootfs and cross-pool cloning doesn't work like that (IIRC)

Plumb the BE root through to libbe(3) so that some things -can- be done to
it, e.g.

bectl -r tank/ROOT create -e default upgrade
bectl -r tank/ROOT mount upgrade /mnt

this aides in some upgrade setups where rootfs is not necessarily ZFS, and
also makes it easier/possible to regression-test bectl when combined with a
file-backed zpool.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D18029