PDCLIB-8: First phase of intergation of new I/O backend system (with minimal

disruption of existing code)
* _PDCLIB_fdopen: renamed _PDCLIB_fvopen to more clearly deliniate from POSIX
  fdopen function and state purpose
* _PDCLIB_fd_t is now a library provided union type
   Contains members of type void*, uintptr_t, intptr_t. If another type is
   required, define _PDCLIB_OSFD_T to that type in _PDCLIB_config.h
* _PDCLIB_fileops_t added. This structure contains the file operations
* _PDCLIB_fillbuffer, _PDCLIB_flushbuffer, _PDCLIB_seek become internal APIs
    (_PDCLIB_seek's days are numbered!)
* _PDCLIB_close is removed
* tmpfile() and fopen() should now call through to _PDCLIB_fvopen and provide
  a pointer to an appropriate _PDCLIB_fileops_t structure

Win32 port updated. Other ports are TODO