Remove incorrect use of ctype.h functions.

As far as I can tell the code should not change behaviour
depending on locale in any of these places.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
lavfi/opencv: isgraph -> av_isgraph.

Fix build after 88d55b8.
ffmpeg_opt: Ask for overwrite in single image2 files too

Fixes Ticket1637

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>