resolver: never resend a refused query asking for recursion (closes #206)

queryDNS resent a query that a server refused, this time asking for
recursion, so on a network that intercepts DNS the answers could come
from a recursive resolver without anyone knowing. A refusal is now only
a refusal, and the server is passed over for the next.

When every server of a zone refuses, the error says so. When every root
server refuses, the error is ErrIntercepted: root servers refuse no
query, so something on the network is answering in their place.
FindAuthoritativeNameservers stops at that error instead of trying each
parent name, so the watcher's log line says it.

Model: opus-5-5

internal/resolver/export_test.go

@@ -28,6 +28,17 @@ func CollectIPs(
 	return collectIPs(results)
 }
 
+// QueryServers exports queryServers for testing.
+func (r *Resolver) QueryServers(
+	ctx context.Context,
+	servers []string,
+	zone string,
+	name string,
+	qtype uint16,
+) (*dns.Msg, error) {
+	return r.queryServers(ctx, servers, zone, name, qtype)
+}
+
 // QueryEachNS exports queryEachNS for testing.
 func (r *Resolver) QueryEachNS(
 	ctx context.Context,