libbe(3): Disambiguate 'active' a little bit, add 'bootfs'

• 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.