Add support for MX, TXT, NS and SOA records to GResolver

* Add resolver functions for looking up DNS records of
   various types. Currently implemented: MX, TXT, SOA, SRV, NS
 * Return records as GVariant tuples.
 * Make the GSrvTarget lookups a wrapper over this new
   functionality.
 * Rework the resolver test so that it has support for
   looking up MX, NS, SOA, TXT records, and uses GOptionContext

https://bugzilla.gnome.org/show_bug.cgi?id=672944

docs/reference/gio/gio-sections.txt

@@ -1699,6 +1699,9 @@ g_resolver_lookup_service
 g_resolver_lookup_service_async
 g_resolver_lookup_service_finish
 g_resolver_free_targets
+g_resolver_lookup_records
+g_resolver_lookup_records_async
+g_resolver_lookup_records_finish
 
 <SUBSECTION>
 G_RESOLVER_ERROR
@@ -1717,6 +1720,7 @@ G_TYPE_RESOLVER
 GResolverPrivate
 g_resolver_get_type
 g_resolver_error_quark
+g_resolver_record_type_get_type
 </SECTION>
 
 <SECTION>