plugin: Prevent plugins from registering native notification topics

They may already have subscribers, and they may crash if presented
with a malformed notification.

lightningd/notification.c

@@ -18,10 +18,15 @@ static struct notification *find_notification_by_topic(const char* topic)
 	return NULL;
 }
 
-bool notifications_have_topic(const struct plugins *plugins, const char *topic)
+bool notifications_topic_is_native(const char *topic)
 {
 	struct notification *noti = find_notification_by_topic(topic);
-	if (noti)
+	return noti != NULL;
+}
+
+bool notifications_have_topic(const struct plugins *plugins, const char *topic)
+{
+	if (notifications_topic_is_native(topic))
 		return true;
 
 	/* Some plugin at some point announced it'd be emitting

lightningd/notification.h

@@ -27,6 +27,10 @@ struct wally_psbt;
 
 bool notifications_have_topic(const struct plugins *plugins, const char *topic);
 
+/* Is the provided notification topic native, i.e., provided by
+ * lightningd itself? */
+bool notifications_topic_is_native(const char *topic);
+
 struct notification {
 	const char *topic;
 	/* the serialization interface */

lightningd/plugin.c

@@ -1327,6 +1327,14 @@ static const char *plugin_notifications_add(const char *buffer,
 				       i);
 
 		name = json_strdup(plugin->plugins, buffer, method);
+
+		if (notifications_topic_is_native(name))
+			return tal_fmt(plugin,
+				       "plugin attempted to register a native "
+				       "notification topic \"%s\", these may "
+				       "however only be sent by lightningd",
+				       name);
+
 		tal_arr_expand(&plugin->plugins->notification_topics, name);
 	}