ZyklonB: use XDG paths by default

Install plugins to /usr/share rather than /usr/lib since they're
arch-independent.  Many precedents can be found for scripted plugins
in /usr/share and fewer for /usr/lib.

Look for plugins in all XDG data directories and repurpose
the "plugin_dir" setting to override this behaviour.

This adds some complexity to the bot but unifies the project.
It might make sense to remove the "plugin_dir" setting.

config.h.in

@@ -2,7 +2,7 @@
 #define CONFIG_H
 
 #define PROGRAM_VERSION "${project_version}"
-#define ZYKLONB_PLUGIN_DIR "${CMAKE_INSTALL_PREFIX}/${zyklonb_plugin_dir}"
+#define LIBDIR "${CMAKE_INSTALL_FULL_LIBDIR}"
 
 #cmakedefine HAVE_READLINE
 #cmakedefine HAVE_EDITLINE