confile: implement config item clear callback

src/lxc/confile.h

@@ -33,12 +33,12 @@ struct lxc_list;
 
 typedef int (*config_set_cb)(const char *, const char *, struct lxc_conf *);
 typedef int (*config_get_cb)(const char *, char *, int, struct lxc_conf *);
-typedef int (*config_clear_cb)(void);
+typedef int (*config_clr_cb)(const char *key, struct lxc_conf *c);
 struct lxc_config_t {
 	char *name;
 	config_set_cb set;
 	config_get_cb get;
-	config_clear_cb clear;
+	config_clr_cb clear;
 };
 
 extern struct lxc_config_t *lxc_getconfig(const char *key);