[patch] Require _KERNEL/_KERNEL_STRUCTURES for all kernel headers

Hi,

I'd like to see fewer 'M's when I cvsup, so I'd like to commit this. 
I've submitted it before, a long time ago, and it was fairly
well-received (Matt liked it.)  I've been running with it ever since and
it hasn't caused me any problems.  It shouldn't break any ports that
aren't already broken; in fact, it should make it easier to see what the
problem is in some cases.  Of course, I'll try to assist in fixing any
ports it breaks, where I can.

I just wanted to pass it over submit@ for one last review.  If there are
no objections in the next few days, I'll commit it:

  http://catseye.webhop.net/DragonFlyBSD/patch/kernel_barricade.diff

Thanks,
-Chris