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