FreeBSD: add support for KSTAT_TYPE_RAW

A few kstats use KSTAT_TYPE_RAW to provide a string generated on
demand. Implementing these as sysctls was punted until now.

Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10836