df-open: add a 'open_commands' list to stash pending opens around in

`openchannel_signed` commands hang out across the openingd/channeld
boundary -- we don't return until we've successfully broadcast the
transaction (or timed out waiting for them to send a tx_sigs back).

lightningd/lightningd.h

@@ -178,6 +178,8 @@ struct lightningd {
 	struct list_head sendpay_commands;
 	/* Outstanding close commands. */
 	struct list_head close_commands;
+	/* Outstanding openchannel_signed commands. */
+	struct list_head open_commands;
 	/* Outstanding ping commands. */
 	struct list_head ping_commands;