xpay: support paying a (simple) bolt12 offer directly.

fetchinvoice is still good for detailed diagnostics and handling
recurring invoices and alternate currencies, but this covers the
"throw some sats" case well.

Changelog-Added: JSON-RPC: `xpay` can now pay a simple offer directly, rather than requiring fetchinvoice first.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/bolt12.c

@@ -594,7 +594,7 @@ static bool bolt12_has_request_prefix(const char *str)
 	return strstarts(str, "lnr1") || strstarts(str, "LNR1");
 }
 
-static bool bolt12_has_offer_prefix(const char *str)
+bool bolt12_has_offer_prefix(const char *str)
 {
 	return strstarts(str, "lno1") || strstarts(str, "LNO1");
 }

common/bolt12.h

@@ -125,6 +125,7 @@ void offer_period_paywindow(const struct recurrence *recurrence,
 /**
  * Preliminary prefix check to see if the string might be a bolt12 string.
  */
+bool bolt12_has_offer_prefix(const char *str);
 bool bolt12_has_prefix(const char *str);
 
 /**