conf{,ile}: warn user once about legacy config

src/lxc/conf.h

@@ -408,6 +408,12 @@ struct lxc_conf {
 
 	/* RLIMIT_* limits */
 	struct lxc_list limits;
+
+	/* REMOVE IN LXC 3.0
+	 * Indicator whether the current config file we're using contained any
+	 * legacy configuration keys.
+	 */
+	bool contains_legacy_key;
 };
 
 #ifdef HAVE_TLS

src/lxc/confile.h

@@ -53,6 +53,7 @@ typedef int (*config_clr_cb)(const char *key, struct lxc_conf *conf,
 
 struct lxc_config_t {
 	char *name;
+	bool is_legacy_key; /* REMOVE in LXC 3.0 */
 	config_set_cb set;
 	config_get_cb get;
 	config_clr_cb clr;