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.

This fixes a segfault in --reload when the image becomes unloadable while feh
is running.

Note that this does not affect --reload with more than one file: If the
current image is removed, feh will simply jump to the next image. feh will
only quit if the file still exists, but can no longer be loaded.

Ugly workaround, gotta make it nicer some time