lib/libbe: open the target boot environment and get a zfs
handle, then pass that with the target mountpoint to be_mount_iter;
If the BE_MNT_DEEP flag is set call zfs_iter_filesystems and mount
the child datasets.
similar logic is employed when unmounting the datasets, save for children
are unmounted first.
sbin/bectl: change bectl_cmd_jail to pass the BE_MNT_DEEP flag when
calling be_mount as well as call be_unount when cleaning up after the
jail has exited.