Sample conformant, clean <c...> headers

[Benjamin Kosnik]

Stephen, thanks for your work and most importantly, for a new idea.

I would like feedback on Stephen's approach. I think it's sound, but am 
interested in hearing from Nathan and others.

Nathan? Care to comment? Scott? 

> With this implementation of the <c...> headers, the whole c_std/c_shadow thing
> becomes completely unnecessary.

> I can whip up some more headers next week so I can try a full bootstrap and
> run with the testsuites.