Splice: Rotating funding pubkey fix

Interop testing with Eclair revealed an issue with remote funding key rotation.

This searches for the funding output using the rotated remote funding pubkey instead of the furrent funding pubkey.

Also update the variable name to be more clear which this represents.

Changelog-Changed: Interop fixes for compatability with Eclair

Author: ddustin

channeld/channeld.c

@@ -3306,7 +3306,7 @@ static void resume_splice_negotiation(struct peer *peer,
 	struct bitcoin_tx *bitcoin_tx;
 	u32 splice_funding_index;
 	const u8 *msg, *sigmsg;
-	u32 chan_output_index;
+	u32 new_output_index;
 	struct pubkey *their_pubkey;
 	struct bitcoin_tx *final_tx;
 	struct bitcoin_txid final_txid;
@@ -3337,8 +3337,8 @@ static void resume_splice_negotiation(struct peer *peer,
 					 &peer->channel->funding_pubkey[LOCAL],
 					 &peer->channel->funding_pubkey[REMOTE]);
 
-	find_channel_output(peer, current_psbt, &chan_output_index,
-			    &peer->channel->funding_pubkey[REMOTE]);
+	find_channel_output(peer, current_psbt, &new_output_index,
+			    &inflight->remote_funding);
 
 	splice_funding_index = find_channel_funding_input(current_psbt,
 							  &peer->channel->funding);
@@ -3622,7 +3622,7 @@ static void resume_splice_negotiation(struct peer *peer,
 
 		final_tx = bitcoin_tx_with_psbt(tmpctx, current_psbt);
 		msg = towire_channeld_splice_confirmed_signed(tmpctx, final_tx,
-							      chan_output_index);
+							      new_output_index);
 		wire_sync_write(MASTER_FD, take(msg));
 	}
 }