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

include/os/freebsd/zfs/sys/arc_os.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ * SPDX-License-Identifier: BSD-2-Clause
  *
  * Copyright (c) 2022 Martin Matuska
  *

include/os/freebsd/zfs/sys/freebsd_event.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ * SPDX-License-Identifier: BSD-2-Clause
  *
  * Copyright (c) 2022 Rob Wing
  *

include/os/freebsd/zfs/sys/zfs_context_os.h

@@ -78,7 +78,7 @@ extern int hz;
 extern int tick;
 typedef int fstrans_cookie_t;
 #define	spl_fstrans_mark() (0)
-#define	spl_fstrans_unmark(x) (x = 0)
+#define	spl_fstrans_unmark(x) ((void)x)
 #define	signal_pending(x) SIGPENDING(x)
 #define	current curthread
 #define	thread_join(x)