opts: Add option to register extra TLV types to accept

Incoming HTLCs are rejected by the HTLC logic if the payload contains
an even type that `lightningd` doesn't recognize. This is to prevent
us from accidentally accepting a payment that has extra semantics
attached (for example if we get a keysend payment and don't know what
to do with the TLV field containing the message we should reject it,
otherwise the overall semantics of the message delivery fail).

lightningd/options.c

@@ -1484,8 +1484,12 @@ static void add_config(struct lightningd *ld,
 			 * --plugin for each one, so ignore these */
 #if EXPERIMENTAL_FEATURES
 		} else if (opt->cb_arg == (void *)opt_set_accept_extra_tlv_types) {
-#endif
                         /* TODO Actually print the option */
+#endif
+#if DEVELOPER
+		} else if (strstarts(name, "dev-")) {
+			/* Ignore dev settings */
+#endif
 		} else {
 			/* Insert more decodes here! */
 			abort();