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
cc624025b4