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

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