resolver: a reply marked recursion available from a root server's address stops the lookup (closes #300)

dnswatcher never asks for recursion and a root server never offers it,
so a reply marked recursion available from a root server's address
came from a resolver on the network answering in its place. The first
such reply now stops the lookup at once with the existing "this network
intercepts DNS queries", naming the address, and no other root server
is asked, whether or not that resolver had the name cached. Before, a
cached name was checked against the resolver's second-hand answer.

On such a network dnswatcher and its tests fail with that error, by
design; nothing works around the interception. A live test passes
public resolvers as the root servers and expects the error.

Model: opus-5-5

internal/resolver/errors.go

@@ -53,9 +53,12 @@ var (
 		"reply truncated and its retry over TCP failed",
 	)
 
-	// ErrIntercepted is returned when every root server refused a
-	// query. Root servers refuse no query, so the refusals came from
-	// something on the network answering in their place.
+	// ErrIntercepted is returned when a root server's address gave a
+	// reply marked recursion available, which ends the lookup at once,
+	// or when every root server refused a query. Root servers do
+	// neither, so those replies came from something on the network
+	// answering in their place. On such a network dnswatcher fails with
+	// it, by design: nothing works around the interception.
 	ErrIntercepted = errors.New("this network intercepts DNS queries")
 
 	// ErrCNAMEDepthExceeded is returned when a CNAME chain