Also refactor `zpool` type to use `zfs_fs` to construct the root dataset
for the zpool, which means we no longer need to duplicate the dataset
create and mount logic inside the `zpool` type.
If the zpool's root dataset is not the rootfs and gets mounted on
creation, the actual rootfs will get mounted later and shadow the
current mountpoint.
Running `zfs unmount` is the easiest way to unmount the zpool's root
dataset on creation without messing up the value of the `mountpoint`
setting.
