>Why is this a good thing to have a sysctl for?
>* it's not gonna change on a given MACHINE_ARCH.  (if it does, well,
>something _else_ is seriously wrong.)
>* it's easier, nicer, faster code is produced if you do 
>masks/shifts/ands etc. with constants known at compile time than if
>you do them all as needed at run-time with variables.

	ALIGNBYTES can be different between the kernel and the userland,
	if you run sparc32 binary on sparc64 kernel.
	http://mail-index.netbsd.org/tech-net/2000/02/28/0012.html
	(and bunch of emails near this one)

itojun