df-rbf: only permit the channel initiator to init RBF

We're *mostly* set up for both sides doing RBF, except that it reverses
the callback flow (using the plugin vs RPC calls) and we're not
currently smart enough to flip between them gracefully

lightningd/dual_open_control.c

@@ -1807,6 +1807,11 @@ json_openchannel_bump(struct command *cmd,
 				    " Current state %s, expected state %s",
 				    channel_state_name(channel),
 				    channel_state_str(DUALOPEND_AWAITING_LOCKIN));
+	if (channel->opener != LOCAL)
+		return command_fail(cmd, FUNDING_STATE_INVALID,
+				    "Only the channel opener can initiate an"
+				    " RBF attempt");
+
 	/* Ok, we're kosher to start */
 	channel->open_attempt = oa = new_channel_open_attempt(channel);
 	oa->funding = *amount;