Eliminating monolithic winsup.h

[Chris Faylor]

I've been slowly moving all of the cygwin header files out of winsup.h.
The goal is to improve compile times and to allow better dependency checking.

I was inspired by DJ's recent windows header file changes.

One thing that I'm not 100% sure about is relying on header file ordering.
Currently, I've tried to keep everything flat.  That means that .h files do
not include other .h files even if they depend on them.

I'm noticing that some header files like sigproc.h and shared.h are still
too monolithic for their own good and probably need to be broken down more.
That will be a later step.

FYI,
cgf