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

tests/zfs-tests/cmd/dosmode_readonly_write.c

@@ -1,5 +1,5 @@
 /*
- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ * SPDX-License-Identifier: BSD-2-Clause
  *
  * Copyright (c) 2021 iXsystems, Inc.
  *