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

Purpose of "--no-recursive" option is disabling effect of "--recursive" option that enabled by user's theme.

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.

this fixes a segfault when combining --scale-down and --thumbnails on a
sufficiently large directory