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

[kevans]

Summary

(I think now is the time to change the API slightly, if I'm going to)

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.

Test Plan

• Poke @philip into removing one set of mounts from his upgrade procedure using bectl -r

[kevans]

create also needs to learn about zpool altroot, or else it screws things up.

[philip]

Thank you! I'll give this patch a try on my crazy rerooted setup later today.