fflush, freadseek: use fseeko, not fseek

fflush, freadseek: use fseeko, not fseek

Follow our own advice: fseek is not safe to use on large files.

* lib/fflush.c (clear_ungetc_buffer_preserving_position)
(clear_ungetc_buffer): Avoid potential problems on large files.
* lib/freadseek.c (freadseek): Likewise.
* modules/freadseek (Depends-on): Add fseeko.
* modules/fseek (configure.ac): Set a witness.
* tests/test-fflush.c (main): Use fseeko.
* tests/test-fpurge.c (fseek): Disable link warning.
* tests/test-freadable.c (fseek): Likewise.
* tests/test-freading.c (fseek): Likewise.
* tests/test-fseeko.c (fseek): Likewise.
* tests/test-ftell.c (fseek): Likewise.
* tests/test-ftello.c (fseek): Likewise.
* tests/test-fwritable.c (fseek): Likewise.
* tests/test-fwriting.c (fseek): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>