l2tp: Control IPv6 data leak prevention option with PPPD noipv6

[l2tp] Control IPv6 data leak prevetion option with PPPD noipv6. JB#53542

Add PPPD supported noipv6 option to supported L2TP options.

Control the IPv6 data leak prevention feature with PPPD option noipv6.
Require this value to be explicitly set in order to use the feature.

connman/vpn/plugins/l2tp.c

@@ -119,6 +119,7 @@ struct {
 	{ "PPPD.ReqMPPE128", "require-mppe-128", OPT_PPPD, NULL, OPT_BOOL },
 	{ "PPPD.ReqMPPEStateful", "mppe-stateful", OPT_PPPD, NULL, OPT_BOOL },
 	{ "PPPD.NoVJ", "novj", OPT_PPPD, NULL, OPT_BOOL },
+	{ "PPPD.NoIPv6", "noipv6", OPT_PPPD, NULL, OPT_BOOL},
 };
 
 static DBusConnection *connection;
@@ -386,6 +387,7 @@ static int write_pppd_option(struct vpn_provider *provider, int fd)
 {
 	int i;
 	const char *opt_s;
+	bool no_ipv6 = false;
 
 	l2tp_write_option(fd, "nodetach", NULL);
 	l2tp_write_option(fd, "lock", NULL);
@@ -409,6 +411,12 @@ static int write_pppd_option(struct vpn_provider *provider, int fd)
 		if (!opt_s)
 			continue;
 
+		if (!g_strcmp0(opt_s, "PPPD.NoIPv6")) {
+			no_ipv6 = vpn_provider_get_boolean(provider,
+						pppd_options[i].cm_opt, false);
+			continue;
+		}
+
 		if (pppd_options[i].type == OPT_STRING)
 			l2tp_write_option(fd,
 				pppd_options[i].pppd_opt, opt_s);
@@ -424,6 +432,8 @@ static int write_pppd_option(struct vpn_provider *provider, int fd)
 	l2tp_write_option(fd, "plugin",
 				SCRIPTDIR "/libppp-plugin.so");
 
+	vpn_provider_set_supported_ip_networks(provider, true, !no_ipv6);
+
 	return 0;
 }