fflush: adjust to glibc 2.28 libio.h removal

fflush: adjust to glibc 2.28 libio.h removal

Problem reported by Daniel P. Berrangé in:
https://lists.gnu.org/r/bug-gnulib/2018-03/msg00000.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):
Check _IO_EOF_SEEN instead of _IO_ftrylockfile.
* lib/stdio-impl.h (_IO_IN_BACKUP) [_IO_EOF_SEEN]:
Define if not already defined.