Update zfs.sh work on FreeBSD

Extend the zfs.sh script to load and unload zfs kmods on FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9746