kmem caches and the cache line size

[Andrew Doran <ad%netbsd.org@localhost>]

Hi,

This zero pads the cache names to be more readable in vmstat output, and
avoids creation of caches where the item size is not a factor or multiple of
the cache line size.

	http://www.netbsd.org/~ad/2020/kmem.diff

This gives me a repeatable 1-2% speed up on compile jobs on my test system,
means that __aligned(COHERENCY_UNIT) can be used more easily in structures
allocated with kmem_alloc(), and permits removal of a bunch of padding and
rounding done during allocation.

Comments?

Cheers,
Andrew