lightningd: move local invoice resolution into invoice.c function.

We're going to make it async, so start by moving the core code into
invoice.c and having that directly call fail/success functions for the
htlc.

We add an extra check in fulfill_htlc() that the HTLC state is correct:
that can't happen now, but may once we're async.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/invoice.h

@@ -1,9 +1,25 @@
 #ifndef LIGHTNING_LIGHTNINGD_INVOICE_H
 #define LIGHTNING_LIGHTNINGD_INVOICE_H
 #include "config.h"
-#include <bitcoin/preimage.h>
-#include <ccan/crypto/sha256/sha256.h>
-#include <ccan/list/list.h>
-#include <ccan/tal/tal.h>
+#include <wire/gen_onion_wire.h>
+
+struct amount_msat;
+struct htlc_in;
+struct lightningd;
+struct sha256;
+
+/**
+ * invoice_try_pay - process payment for this payment_hash, amount msat.
+ * @ld: lightningd
+ * @hin: the input HTLC which is offering to pay.
+ * @payment_hash: hash of preimage they want.
+ * @msat: amount they offer to pay.
+ *
+ * Either calls fulfill_htlc() or fail_htlcs().
+ */
+void invoice_try_pay(struct lightningd *ld,
+		     struct htlc_in *hin,
+		     const struct sha256 *payment_hash,
+		     const struct amount_msat msat);
 
 #endif /* LIGHTNING_LIGHTNINGD_INVOICE_H */