The strncpy function does not guarantee to end the resulting character
sequence with a terminating nul character if not enough space is
available. This could be triggered by supplying a sufficiently long
output_file option.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

manually changed in collage, imlib, filelist, thumbnail, slideshow,
index
feh_display_status also changed

It's all documented in the manpage anyways.

Centering didn't work if the text below the thumbnail was wider than the
thumbnail itself. Fixed.

This fixes imlib2 warnings (and occasional X errors/segfaults), which used to
occur when all files in feh's filelist were suddenly deleted by an external
application. Now, feh simply quits with "No more slides in show" (which makes
more sense, IMHO).