This makes us more resilient to a rename of the bootfs, but still wouldn't
withstand pool renames or guid renames. More importantly, this allows `bectl
create <foo>` work out of the box to create a boot environment based on the
currently booted one.
- Rename 'active' to 'rootfs', which is used in other places to describe the
currently booted (or about to be booted) BE.
- Add 'bootfs', which indicates the next boot environment to be booted. This
is pulled from the BOOTFS zpool property.
- Go ahead and keep an open handle to the active zpool. We might need to
enumerate datasets, get properties, and set properties (e.g. bootfs)
throughout other libbe bits, and a single handle isn't overly expensive.
