tls-peer: Handle HelloRetryRequest

Adds support to handle retries with different DH group and/or a cookie
extension.

src/libtls/tls.h

@@ -189,6 +189,12 @@ enum tls_name_type_t {
  */
 extern enum_name_t *tls_extension_names;
 
+/**
+ * Magic value (SHA-256 of "HelloRetryRequest") for Random to differentiate
+ * ServerHello from HelloRetryRequest.
+ */
+extern chunk_t tls_hello_retry_request_magic;
+
 /**
  * A bottom-up driven TLS stack, suitable for EAP implementations.
  */