FreeBSD: Add support for _PC_HAS_HIDDENSYSTEM

In FreeBSD there is now a pathconf name _PC_HAS_HIDDENSYSTEM.
This patch adds support for it to OpenZFS.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Closes #17518