config-policy: push endpoints with keep=true to the end of the list when sorting

This ensures that endpoints with keep=false will still have
a chance to link when ones with keep=true exist. This also effectively
means that role priority does not matter when keep=true: we keep these
links active no matter what, while policy still applies normally for
all others.

For example, a final sorted list with these endpoints will end up
looking like this:

* ep1, role priority=25, keep=false
* ep2, role priority=20, keep=false
* ep3, role priority=25, keep=true
* ep4, role priority=75, keep=true

... which will effectively cause ep1, ep3 and ep4 to be linked.

modules/module-config-policy/config-policy.c

@@ -304,6 +304,10 @@ link_info_compare_func (gconstpointer a, gconstpointer b, gpointer data)
   guint priority_b = 0;
   gint res = 0;
 
+  res = (gint) li_a->keep - (gint) li_b->keep;
+  if (res != 0)
+    return res;
+
   /* Get the role priority of a */
   stream_a = wp_base_endpoint_get_stream (target, li_a->stream_id);
   if (stream_a)