resolver: pass over a server that answers SERVFAIL or refers no closer (closes #197)

When the resolver walks from the root servers towards a name, a server
that answered SERVFAIL, or referred the query back to its own zone, up
or sideways, ended the step, so finding a zone's servers gave up on the
zone though its other servers would answer. Such a reply is now passed
over for the zone's next server, as a timeout or a refusal already was.
To tell a referral that leads closer to the name from one that does
not, each walk keeps the zone of the servers it is asking. Other error
replies, such as FORMERR, are passed over too. The walk that finds a
nameserver's address shares the same server loop, so it changes too.

Model: opus-5-5

internal/resolver/export_test.go

@@ -11,6 +11,11 @@ func ExtractRecordValue(rr dns.RR) string {
 	return extractRecordValue(rr)
 }
 
+// UsableReply exports usableReply for testing.
+func UsableReply(resp *dns.Msg, zone string, name string) bool {
+	return usableReply(resp, zone, name)
+}
+
 // CollectIPs exports collectIPs for testing.
 func CollectIPs(
 	results map[string]*NameserverResponse,