paymod: Check if destination is reachable at all directly at startup

This does two things: it checks if the destination of the payment is at all
reachable without routehints, and if it is it adds a direct attempt as option
to the routehints in the form of a NULL routehint. It also simplifies the
selection of the routehint since the direct case is no longer special, instead
we just return a NULL routehint as if it were a normal routehint.

plugins/libplugin-pay.h

@@ -299,6 +299,9 @@ struct routehints_data {
 	/* We modify the CLTV in the getroute call, so we need to remember
 	 * what the final cltv delta was so we re-apply it correctly. */
 	u32 final_cltv;
+
+	/* Is the destination reachable without any routehints? */
+	bool destination_reachable;
 };
 
 struct exemptfee_data {