Rework os resolver

Currently, os resolver not only handle A and AAAA records, but also does
it wrongly, since when it packs AAAA record to a dns.A record.

This commit reworks os resolver to make it works with all supported
record types.

cmd/ctrld/dns_proxy.go

@@ -296,5 +296,5 @@ func ttlFromMsg(msg *dns.Msg) uint32 {
 
 var osUpstreamConfig = &ctrld.UpstreamConfig{
 	Name: "OS resolver",
-	Type: "os",
+	Type: ctrld.ResolverTypeOS,
 }