Port extended stdio modules to HP NonStop Kernel.

* lib/stdio-impl.h (_IOERR, _IOREAD, _IOWRT, _IORW) [__TANDEM]: New
macros.
* lib/fbufmode.c: Update comments.
* lib/fflush.c: Likewise.
* lib/fpurge.c: Likewise.
* lib/freadable.c: Likewise.
* lib/freadahead.c: Likewise.
* lib/freading.c: Likewise.
* lib/freadptr.c: Likewise.
* lib/freadseek.c: Likewise.
* lib/fseeko.c: Likewise.
* lib/fseterr.c: Likewise.
* lib/fwritable.c: Likewise.
* lib/fwriting.c: Likewise.
Reported by Joachim Schmitz <jojo@schmitz-digital.de>.