[settings] Avoid duplicate settings block names

Automatically unregister any settings with the same name (and position
within the settings tree) as a newly registered settings block.

This functionality is generalised out from dhcp.c.

src/core/settings.c

  * @ret rc		Return status code
  */
 int register_settings ( struct settings *settings, struct settings *parent ) {
+	struct settings *old_settings;
 
 	/* NULL parent => add to settings root */
 	assert ( settings != NULL );
 	if ( parent == NULL )
 		parent = &settings_root;
 
+	/* Remove any existing settings with the same name */
+	if ( ( old_settings = find_child_settings ( parent, settings->name ) ))
+		unregister_settings ( old_settings );
+
 	/* Add to list of settings */
 	ref_get ( settings->refcnt );
 	ref_get ( parent->refcnt );