paymod: Add a COMPAT_090 flag

Since we end up consolidating some of the return values for `pay` and
`paystatus` and change the public interface we need to add the compatibility
flag and guard the switchover behind it.

Makefile

@@ -65,7 +65,7 @@ endif
 
 ifeq ($(COMPAT),1)
 # We support compatibility with pre-0.6.
-COMPAT_CFLAGS=-DCOMPAT_V052=1 -DCOMPAT_V060=1 -DCOMPAT_V061=1 -DCOMPAT_V062=1 -DCOMPAT_V070=1 -DCOMPAT_V072=1 -DCOMPAT_V073=1 -DCOMPAT_V080=1 -DCOMPAT_V081=1 -DCOMPAT_V082=1
+COMPAT_CFLAGS=-DCOMPAT_V052=1 -DCOMPAT_V060=1 -DCOMPAT_V061=1 -DCOMPAT_V062=1 -DCOMPAT_V070=1 -DCOMPAT_V072=1 -DCOMPAT_V073=1 -DCOMPAT_V080=1 -DCOMPAT_V081=1 -DCOMPAT_V082=1 -DCOMPAT_V090=1
 endif
 
 # Timeout shortly before the 600 second travis silence timeout

plugins/pay.c

@@ -1269,6 +1269,9 @@ static struct pay_status *add_pay_status(struct pay_command *pc,
 	return ps;
 }
 
+#ifndef COMPAT_V090
+UNUSED
+#endif
 static struct command_result *json_pay(struct command *cmd,
 				       const char *buf,
 				       const jsmntok_t *params)
@@ -1705,13 +1708,15 @@ static void init(struct plugin *p,
 	maxdelay_default = atoi(field);
 }
 
-#if DEVELOPER
 struct payment_modifier *paymod_mods[3] = {
 	&dummy_pay_mod,
 	&retry_pay_mod,
 	NULL,
 };
 
+#if !DEVELOPER
+UNUSED
+#endif
 static struct command_result *json_paymod(struct command *cmd,
 					  const char *buf,
 					  const jsmntok_t *params)
@@ -1770,7 +1775,6 @@ static struct command_result *json_paymod(struct command *cmd,
 
 	return command_still_pending(cmd);
 }
-#endif
 
 static const struct plugin_command commands[] = { {
 		"pay",