gluon-respondd: emit "primary_domain_code" in nodeinfo

Closes #1974

Situation:
==========

$ ls -l /lib/gluon/domains/lindennord.json
lrwxrwxrwx    1 root     root            10 Jan  6 03:42 /lib/gluon/domains/lindennord.json -> dom17.json

Before:
=======

$ gluon-neighbour-info -d ::1 -p 1001 -r nodeinfo -c 1
{
   "node_id": "525400123456",
   "system": {
     "domain_code": "lindennord",
     "site_code": "ffh"
   },
...
}

After:
======

$ gluon-neighbour-info -d ::1 -p 1001 -r nodeinfo -c 1
{
   "node_id": "525400123456",
   "system": {
     "primary_domain_code": "dom17",
     "domain_code": "lindennord",
     "site_code": "ffh"
   },
...
}

package/gluon-respondd/src/respondd-nodeinfo.c

@@ -64,6 +64,10 @@ static struct json_object * get_site_code(void) {
 	return ret;
 }
 
+static struct json_object * get_primary_domain_code(void) {
+	return gluonutil_wrap_and_free_string(gluonutil_get_primary_domain());
+}
+
 static struct json_object * get_domain_code(void) {
 	return gluonutil_wrap_and_free_string(gluonutil_get_domain());
 }
@@ -124,8 +128,10 @@ struct json_object * respondd_provider_nodeinfo(void) {
 
 	struct json_object *system = json_object_new_object();
 	json_object_object_add(system, "site_code", get_site_code());
-	if (gluonutil_has_domains())
+	if (gluonutil_has_domains()) {
 		json_object_object_add(system, "domain_code", get_domain_code());
+		json_object_object_add(system, "primary_domain_code", get_primary_domain_code());
+	}
 	json_object_object_add(ret, "system", system);
 
 	return ret;