spa: Let spa_taskq_param_get()'s addition of a newline be optional

For FreeBSD sysctls, we don't want the extra newline, since the
sysctl(8) utility will format strings appropriately.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reported-by: Peter Holm <pho@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #15719