experimental-dual-fund: add config level option for enabling dual-fund

Allow a user to switch on dual-funding without needing to compile
as EXPERIMENTAL_FEATURES.

Doesn't work yet, since everything is still behind
'EXPERIMENTAL_FEATURES' compile time flags... but useful for testing

common/features.h

@@ -113,16 +113,16 @@ u8 *featurebits_or(const tal_t *ctx, const u8 *f1 TAKES, const u8 *f2 TAKES);
 #define OPT_LARGE_CHANNELS			18
 #define OPT_ANCHOR_OUTPUTS			20
 
+/* BOLT-7b04b1461739c5036add61782d58ac490842d98b #9:
+ * | 222/223 | `option_dual_fund` | ... IN9 ...
+ */
+#define OPT_DUAL_FUND 				222
+
 /* BOLT-9fc25cfd2895578c0b1ab701ebe6c1eb67a19623 #9:
  *
  * | 102/103 | `option_onion_messages` |... INC+ ...
  */
 #if EXPERIMENTAL_FEATURES
 #define OPT_ONION_MESSAGES			102
-
-/* BOLT-7b04b1461739c5036add61782d58ac490842d98b #9:
- * | 222/223 | `option_dual_fund` | ... IN9 ...
- */
-#define OPT_DUAL_FUND 				222
 #endif
 #endif /* LIGHTNING_COMMON_FEATURES_H */

lightningd/options.c

@@ -791,6 +791,14 @@ static char *opt_set_wumbo(struct lightningd *ld)
 	return NULL;
 }
 
+static char *opt_set_dual_fund(struct lightningd *ld)
+{
+	feature_set_or(ld->our_features,
+		       take(feature_set_for_feature(NULL,
+						    OPTIONAL_FEATURE(OPT_DUAL_FUND))));
+	return NULL;
+}
+
 static void register_opts(struct lightningd *ld)
 {
 	/* This happens before plugins started */
@@ -832,6 +840,13 @@ static void register_opts(struct lightningd *ld)
 				 opt_set_wumbo, ld,
 				 "Allow channels larger than 0.16777215 BTC");
 
+	opt_register_early_noarg("--experimental-dual-fund",
+				 opt_set_dual_fund, ld,
+				 "experimental: Advertise dual-funding"
+				 " and allow peers to establish channels"
+				 " via v2 channel open protocol");
+
+
 	opt_register_noarg("--help|-h", opt_lightningd_usage, ld,
 				 "Print this message.");
 	opt_register_arg("--rgb", opt_set_rgb, NULL, ld,
@@ -1242,6 +1257,11 @@ static void add_config(struct lightningd *ld,
 				      feature_offered(ld->our_features
 						      ->bits[INIT_FEATURE],
 						      OPT_LARGE_CHANNELS));
+		} else if (opt->cb == (void *)opt_set_dual_fund) {
+			json_add_bool(response, name0,
+				      feature_offered(ld->our_features
+						      ->bits[INIT_FEATURE],
+						      OPT_DUAL_FUND));
 		} else if (opt->cb == (void *)plugin_opt_flag_set) {
 			/* Noop, they will get added below along with the
 			 * OPT_HASARG options. */