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/sys/zfs_context.h

@@ -695,8 +695,10 @@ extern char *kmem_asprintf(const char *fmt, ...);
 #define	kmem_strfree(str) kmem_free((str), strlen(str) + 1)
 #define	kmem_strdup(s)  strdup(s)
 
+#ifndef __cplusplus
 extern int kmem_scnprintf(char *restrict str, size_t size,
     const char *restrict fmt, ...);
+#endif
 
 /*
  * Hostname information