fix: remove hidden global state

dns.HandleFunc(".", handler) modifies the global dns.DefaultServeMux and
dns.Server uses that if it is initialized without a Handler. This would
cause issues when multiple instances of the module try to Present() at
the same time.

stub.go

@@ -116,9 +116,12 @@ func (s *StubDNS) Present(ctx context.Context, challenge acme.Challenge) error {
 
 	// spawn the server
 	handler := s.make_handler(fqdn, content)
-	// could also use fqdn as pattern, but "." allows logging invalid requests
+	server := &dns.Server{
-	dns.HandleFunc(".", handler)
+		Addr:       s.Address,
-	server := &dns.Server{Addr: s.Address, Net: "udp", TsigSecret: nil}
+		Net:        "udp",
+		Handler:    handler,
+		TsigSecret: nil,
+	}
 	go s.serve(server)
 
 	// store the server for shutdown later