altcp_tls: some fixes

- added `altcp_tls_free_config()`.
- added `altcp_tls_context()` function to allow mbedtls parameter tweak.

  Since state structure isn't exported, this allow application to get
  internal context (port dependent) to tweak it.

- free altcp_pcb when lower error callback called.

src/include/lwip/altcp_tls.h

@@ -72,11 +72,22 @@ struct altcp_tls_config *altcp_tls_create_config_server_privkey_cert(const u8_t
  */
 struct altcp_tls_config *altcp_tls_create_config_client(const u8_t *cert, size_t cert_len);
 
+/** @ingroup altcp_tls
+ * Free an ALTCP_TLS configuration handle
+ */
+void altcp_tls_free_config(struct altcp_tls_config *conf);
+
 /** @ingroup altcp_tls
  * Create new ALTCP_TLS layer
  */
 struct altcp_pcb *altcp_tls_new(struct altcp_tls_config* config, struct altcp_pcb *inner_pcb);
 
+/** @ingroup altcp_tls
+ * Return pointer to internal TLS context so application can tweak it.
+ * Real type depends on port (e.g. mbedtls)
+ */
+void *altcp_tls_context (struct altcp_pcb *conn);
+
 #ifdef __cplusplus
 }
 #endif