add variadic format checking

Third version with a few minor fixes, the only issues seem to be
with the complexities of the linux-kernel which has some printk
fun locally defined.


This series (which was initially sent back in 2020) adds the
ability to deal with __attribute__((format) and checking the
arguments to formatted variadic functions.

I have been considering adding a -Wformat-linux to this as
the kernel now has a number of extra formatting options and
checking the type of these would be useful. An even nicer
extension would be some way of informing the compiler/sparse
of these at compile time (but would need to get agreement on
how to do this with the compilers too)