updated variadic printf/scanf checking

I'm reposting this as an RFC as there has been quite a bit of work on
this to get scanf in and to try and simplify the code, fix a few bugs
found when checking the kernel. Depending on the reviews I will post
this for inclusion.

I am also looking at some work to try and make printf and scanf extensible
with the interesting types the kernel adds to printk, either by a new flag
or by some sort of attribute. This is not included in this set.

I think the tests need some more work on scanf, otherwise it seems to work
well with the kernel, and not really found any further issues other than
the odd signed/unsigned and type.