common: use struct onionreply.

This makes it clear we're dealing with a message which is a wrapped error
reply (needing unwrap_onionreply), not an already-wrapped one.

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

devtools/Makefile

@@ -23,6 +23,7 @@ DEVTOOLS_COMMON_OBJS :=				\
 	common/memleak.o			\
 	common/node_id.o			\
 	common/onion.o				\
+	common/onionreply.o			\
 	common/per_peer_state.o			\
 	common/pseudorand.o			\
 	common/json.o				\