plugins/pay: always include bolt11 (and description) in listpays.

We were setting it on the root, but that doesn't get handed to
sendpay.  Our schema doesn't *require* bolt11, either, so this was
missed (there could be a *bolt12* instead).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: JSON-RPC: `listpays` always includes `bolt11` or `bolt12` field.

plugins/libplugin-pay.h

@@ -255,6 +255,10 @@ struct payment {
 	 * true. Set only on the root payment. */
 	bool abort;
 
+	/* We only set invstring/description on one of our sendpays per group,
+	 * so we track when we've done that. */
+	bool invstring_used;
+
 	/* Serialized bolt11/12 string, kept attachd to the root so we can filter
 	 * by the invoice. */
 	const char *invstring;