Fixes non-random behaviour when randomizing file lists several times per
second.

Closes #349

This introduces a new feh_should_ignore_image function which is called
at appropriate places in those modes to skip images which are loadable but
undesired.

* Prevents nasty loading loops

* Prevents zombie subprocesses

* Fixes the conversion timeout detection routine

This option allows you to change the default imlib2 image cache size of 4 MiB.

Similar to the `curl` command-line tool.

The check if buffer == NULL is always false, since buffer is an
autoamtic variable allocated when entering the function.  What we
instead want to do is to check if the string is empty after the call to
exif_get_info(), since that means we could not read any exif
information.
When the code once more is enabled, I discovered that we need to copy
the information string into info_buf as well as into  buffer, since it
is the former that is used to print the exif information on top of the
picture.  Without this change, imlib warns about trying to write NULL
strings.

(cf #309)

* Added an --insecure option to disable host/peer verification in libcurl.
This allows feh to load images on HTTPS hosts with self-signed certificates.
For security, this is not the default.

* Fixed an include in show_usage.

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

This means it will be treated like an HTTP image (no write operations,
copying to /tmp etc), but it's still better than no file:// support.

Also, this is why it's not documented yet.