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>.