Make sure zfs_enable="YES" when ZFS isn't mounted automatically

Make sure to set zfs_enabled=“YES” in /etc/rc.conf when ZFS pool excluding root pool isn't mounted automatically at system boot up.

The root pool is mounted automatically if the setting dosen't exist, so it it difficult to find the problem. I checked canmount and mountpoint properties of unmounted ZFS pool, but they were okay. I had trouble detecting its reason.

We should be extra careful of it in case we manually install the FreeBSD without bsdinstall.