add ability to use syscall debug via kernhist.

[matthew green <mrg%eterna.com.au@localhost>]

hi.

i'd been wanting to get this working for a while, and i've finally
gotten this to the point it works properly:

	http://www.netbsd.org/~mrg/scdebug_kernhist.diff

it's kind of ugly due to the limitations of kernhist, but it already
has proven useful to me, and the ugliness is limited to kern_xxx.c.


i plan to commit this sometime soon.  reviews most welcome.


.mrg.