Create new function ListenAndServeTLS to easy run a DNS server with TLS support

Using the ListenAndServe with network as "tcp-tls" will cause an error, as the
certificates weren't informed. To solve that we created the function
ListenAndServeTLS that will configure a DNS server listening TCP and handling
requests on incoming TLS connections.

See #297

server.go

@@ -93,13 +93,38 @@ func HandleFailed(w ResponseWriter, r *Msg) {
 
 func failedHandler() Handler { return HandlerFunc(HandleFailed) }
 
-// ListenAndServe Starts a server on addresss and network speficied. Invoke handler
+// ListenAndServe Starts a server on address and network specified Invoke handler
 // for incoming queries.
 func ListenAndServe(addr string, network string, handler Handler) error {
 	server := &Server{Addr: addr, Net: network, Handler: handler}
 	return server.ListenAndServe()
 }
 
+// ListenAndServeTLS acts identically to ListenAndServe, except that it
+// expects TLS connections. Additionally, files containing a certificate and
+// matching private key for the server must be provided. If the certificate
+// is signed by a certificate authority, the certFile should be the concatenation
+// of the server's certificate, any intermediates, and the CA's certificate.
+func ListenAndServeTLS(addr, certFile, keyFile string, handler Handler) error {
+	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
+	if err != nil {
+		return err
+	}
+
+	config := tls.Config{
+		Certificates: []tls.Certificate{cert},
+	}
+
+	server := &Server{
+		Addr:      addr,
+		Net:       "tcp-tls",
+		TLSConfig: &config,
+		Handler:   handler,
+	}
+
+	return server.ListenAndServe()
+}
+
 // ActivateAndServe activates a server with a listener from systemd,
 // l and p should not both be non-nil.
 // If both l and p are not nil only p will be used.