Explicitly add opensolaris_load="YES" to loader.conf through the
installer when installing the system on a ZFS root filesystem.

For arm64, zfs_load="YES" does not add opensolaris.ko as a kld
dependency, so add it explicitly to prevent boot-time failures
out-of-box.

PR: 240478
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC (Netgate)