stdioext: Add tentative support for Plan9.

stdioext: Add tentative support for Plan9.

* lib/stdio-impl.h: Include <errno.h>.
* lib/fseterr.c (fseterr) [EPLAN9]: Add conditional code.
* lib/freadable.c (freadable): Likewise.
* lib/fwritable.c (fwritable): Likewise.
* lib/fbufmode.c (fbufmode): Likewise.
* lib/freading.c (freading): Likewise.
* lib/fwriting.c (fwriting): Likewise.
* lib/freadptr.c (freadptr): Likewise.
* lib/freadseek.c (freadptrinc): Likewise.
* lib/freadahead.c (freadahead): Likewise.
* lib/fpurge.c (fpurge): Likewise.
* lib/fseeko.c (rpl_fseeko): Likewise.
* m4/fpending.m4 (gl_PREREQ_FPENDING): Add a variant for Plan9.
Reported by Jens Staal <staal1978@gmail.com>.