fflush: be more paranoid about libio.h change

fflush: be more paranoid about libio.h change

Suggested by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-03/msg00270.html
* lib/fbufmode.c (fbufmode):
* lib/fflush.c (clear_ungetc_buffer_preserving_position)
(disable_seek_optimization, rpl_fflush):
* lib/fpending.c (__fpending):
* lib/fpurge.c (fpurge):
* lib/freadable.c (freadable):
* lib/freadahead.c (freadahead):
* lib/freading.c (freading):
* lib/freadptr.c (freadptr):
* lib/freadseek.c (freadptrinc):
* lib/fseeko.c (fseeko):
* lib/fseterr.c (fseterr):
* lib/fwritable.c (fwritable):
* lib/fwriting.c (fwriting):
Look at _IO_ftrylockfile as well as at _IO_EOF_SEEN.

(cherry picked from gnulib commit 74d9d6a293d7462dea8f83e7fc5ac792e956a0ad)