Fix the FreeBSD userspace build (#15716)

• Mark some parameters to zpool_power*() as unused.
• Add a stub zpool_disk_wait().

Fixes: a9520e6e5 ("zpool: Add slot power control, print power status")

Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>