libbe(3): Check that dataset is to be mounted at / for be_exists

libbe(3): Check that dataset is to be mounted at / for be_exists

This makes the be_exists behavior match the comments that assert that we've
already checked that the dataset derived from the BE name is set to mount at
/.

Other changes of note:
- bectl_list sees another change; changing mountpoint based on mount status
  turns out to be a bad idea, so instead make the mounted property of the
  returned nvlist the path that it's mounted at

- Always return the "mountpoint" property in "mountpoint" if it's ste