FreeBSD: Reduce divergence from in-tree sources

This includes random small tweaks, primarily a build fixes, required
when ZFS is built as part of FreeBSD base.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15368