df-open: use channel_id for openchannel_update and openchannel_signed

Be as specific as possible is a good rule for things, I think

lightningd/opening_common.h

@@ -4,6 +4,7 @@
 #include <bitcoin/pubkey.h>
 #include <common/amount.h>
 #include <common/channel_config.h>
+#include <common/channel_id.h>
 #include <common/derive_basepoints.h>
 #include <common/status_levels.h>
 
@@ -27,6 +28,9 @@ struct uncommitted_channel {
 	/* Reserved dbid for if we become a real struct channel */
 	u64 dbid;
 
+	/* Channel id, v2 opens only */
+	struct channel_id cid;
+
 	/* For logging */
 	struct log *log;