Add server extension parsing

Only accept EC J-PAKE ciphersuite if extension was present and OK (single flag
for both), and ignore extension if we have no password.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index d9e3e5c..b74cca2 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -150,6 +150,7 @@
  * of state of the renegotiation flag, so no indicator is required)
  */
 #define MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS_PRESENT (1 << 0)
+#define MBEDTLS_TLS_EXT_ECJPAKE_KKPP_OK                 (1 << 1)
 
 #ifdef __cplusplus
 extern "C" {