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

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -5428,6 +5428,11 @@ zfs_freebsd_pathconf(struct vop_pathconf_args *ap)
 			return (0);
 		}
 		return (EINVAL);
+#ifdef _PC_HAS_HIDDENSYSTEM
+	case _PC_HAS_HIDDENSYSTEM:
+		*ap->a_retval = 1;
+		return (0);
+#endif
 	default:
 		return (vop_stdpathconf(ap));
 	}