channel_fallen_behind: break out functionality into two methods

We'll need this for moving 'waiting for locked' into dualopend.

lightningd/channel_control.c

@@ -261,16 +261,8 @@ static void peer_got_shutdown(struct channel *channel, const u8 *msg)
 	wallet_channel_save(ld->wallet, channel);
 }
 
-static void channel_fail_fallen_behind(struct channel *channel, const u8 *msg)
+void channel_fallen_behind(struct channel *channel, const u8 *msg)
 {
-	if (!fromwire_channeld_fail_fallen_behind(channel, msg,
-						 cast_const2(struct pubkey **,
-							    &channel->future_per_commitment_point))) {
-		channel_internal_error(channel,
-				       "bad channel_fail_fallen_behind %s",
-				       tal_hex(tmpctx, msg));
-		return;
-	}
 
 	/* per_commitment_point is NULL if option_static_remotekey, but we
 	 * use its presence as a flag so set it any valid key in that case. */
@@ -293,6 +285,21 @@ static void channel_fail_fallen_behind(struct channel *channel, const u8 *msg)
 			       "Awaiting unilateral close");
 }
 
+static void
+channel_fail_fallen_behind(struct channel *channel, const u8 *msg)
+{
+	if (!fromwire_channeld_fail_fallen_behind(channel, msg,
+						 cast_const2(struct pubkey **,
+							    &channel->future_per_commitment_point))) {
+		channel_internal_error(channel,
+				       "bad channel_fail_fallen_behind %s",
+				       tal_hex(tmpctx, msg));
+		return;
+	}
+
+        channel_fallen_behind(channel, msg);
+}
+
 static void peer_start_closingd_after_shutdown(struct channel *channel,
 					       const u8 *msg,
 					       const int *fds)

lightningd/channel_control.h

@@ -38,4 +38,7 @@ void channel_record_open(struct channel *channel);
 /* Forget a channel. Deletes the channel and handles all
  * associated waiting commands, if present. Notifies peer if available */
 void forget_channel(struct channel *channel, const char *err_msg);
+
+/* A channel has unrecoverably fallen behind */
+void channel_fallen_behind(struct channel *channel, const u8 *msg);
 #endif /* LIGHTNING_LIGHTNINGD_CHANNEL_CONTROL_H */