libplugin: add data pointer for plugin convenience.

This avoids globals (and means memleak traverses the variables!): we
only change over the test plugin though, to avoid unnecessary churn.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/keysend.c

@@ -594,7 +594,7 @@ int main(int argc, char *argv[])
 		features->bits[i] = tal_arr(features, u8, 0);
 	set_feature_bit(&features->bits[NODE_ANNOUNCE_FEATURE], KEYSEND_FEATUREBIT);
 
-	plugin_main(argv, init, PLUGIN_STATIC, true, features, commands,
+	plugin_main(argv, init, NULL, PLUGIN_STATIC, true, features, commands,
 		    ARRAY_SIZE(commands), NULL, 0, hooks, ARRAY_SIZE(hooks),
 		    notification_topics, ARRAY_SIZE(notification_topics), NULL);
 }