bectl(8): Sort BEs lightly by active (now or later) BE, then others

While it could be preferred to do this at insert in libbe(3), there's no
convenient way to insert at the head of an nvlist. Instead, we'll make two
passes over- once to print anything active either now or at nextboot, and
another to print everything else.

This doesn't actually impact performance in a significant way here, so we'll
worry about further optimizations if the need actually arises.