Addresses part of issue #75

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 used to save the thumbnail for /path/to/./pmage.png, now it is always
/path/to/image.png

Problems so far:
* leaks zombie processes
* does not work when terminating feh with a signal (since the convert process
  is no longer in feh's process group)

It's all documented in the manpage anyways.

Only if a previously selected item (mouse over, keyboard) is available
though.

This was broken by the changes allowing --reload to retry ad infinitum.

Now force_new is used to decide whether or not to work around the Imlib2
caching issue.  Because of this, a few feh_reload_image calls needed to be
changed.

This reverts commit 8b79bc33.