Split sysutil.h

Move the platform-related macros to a new platform.h.  I make sysutil.h #include platform.h, so this should require no 3rd party libraries that use sysutil.h to break their source compatibility.

Along the way:

* Add OIIO_FORCEINLINE

* Add OIIO_ALLOCA -- same as ALLOCA, but less namespace-polluting. The hope is eventually to phase out the old ALLOCA.

* Get rid of a bunch of unnecessary #include directives in other files.