stdioext: Port to native Windows with MSVC. * lib/stdio-impl.h (WINDOWS_OPAQUE_FILE): New macro. (struct _gl_real_FILE): New type. (fp_, _IOREAD, _IOWRT, _IORW, _IOEOF, _IOERR): New macros, for native Windows. * lib/fbufmode.c (fbufmode): Add code for native Windows. * lib/fflush.c (clear_ungetc_buffer): Treat native Windows like the other SystemV derived implementations. * lib/fpurge.c (fpurge): Likewise. * lib/freadable.c (freadable): Likewise. * lib/freadahead.c (freadahead): Likewise. * lib/freading.c (freading): Likewise. * lib/freadptr.c (freadptr): Likewise. * lib/freadseek.c (freadptrinc): Likewise. * lib/fseeko.c (fseeko): Likewise. * lib/fseterr.c (fseterr): Likewise. * lib/fwritable.c (fwritable): Likewise. * lib/fwriting.c (fwriting): Likewise. Reported by Gisle Vanem <gvanem@yahoo.no>.