FreeBSD: make adjustments for the standalone environment

In FreeBSD, there are three compile environments that are supported:
user land, the kernel and the bootloader / standalone. Adjust the
headers to compile in the standalone environment. Limit kernel-only
items from view when _STANDALONE is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Warner Losh <imp@FreeBSD.org>
Closes #10998

include/os/freebsd/spl/sys/proc.h

@@ -41,7 +41,7 @@
 #include <sys/kmem.h>
 #include <sys/malloc.h>
 
-
+#ifdef _KERNEL
 #define	CPU		curcpu
 #define	minclsyspri	PRIBIO
 #define	defclsyspri minclsyspri
@@ -111,4 +111,5 @@ zfs_proc_is_caller(proc_t *p)
 	return (p == curproc);
 }
 
+#endif	/* _KERNEL */
 #endif	/* _OPENSOLARIS_SYS_PROC_H_ */