Refactored plugin-loader with improved dependency resolution

With the new implementation the plugins don't have to be listed in any
special order, dependencies are properly resolved.  The order only
matters if two plugins provide the same feature.

src/libstrongswan/plugins/plugin_loader.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2012 Tobias Brunner
+ * Copyright (C) 2012-2013 Tobias Brunner
  * Copyright (C) 2007 Martin Willi
  * Hochschule fuer Technik Rapperswil
  *
@@ -84,9 +84,9 @@ struct plugin_loader_t {
 	/**
 	 * Create an enumerator over all loaded plugins.
 	 *
-	 * In addition to the plugin, the enumerator returns a list of pointers to
-	 * plugin features currently loaded (if the argument is not NULL).
-	 * This list is to be read only.
+	 * In addition to the plugin, the enumerator optionally provides a list of
+	 * pointers to plugin features currently loaded.
+	 * This list has to be destroyed.
 	 *
 	 * @return				enumerator over plugin_t*, linked_list_t*
 	 */