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
This is to accomodate a later change in libbe(3) that will always return the
mountpoint, whether it be the directory the dataset is actively mounted at
or the "mountpoint" property.
