Add description of option "--no-recursive" to man page.

Turns out giflib 5.1.2 is the culprit and it isn't Deban-specific after all.

This should also help distributions working towards reproducible builds.

* 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.

The following adds the ability, when using `feh` as a background setter,
to use a specific Xinerama screen to render a provided image. Along with
the provided '--bg-*' option, the '--xinerama-index' argument will be
used to identify the index of the detected Xinerama screen to use.

Signed-off-by: James Knight <james.d.knight@live.com>

While at it: Add some notes about .fehbg in general and the fact that it is
an executable shell script now.

Sort filelist by modification time, newer files first. Useful for
perusing a directory of images by most recently added or changed.