Clean up existing SSL restartable ECC code

- more consistent naming with ecrs prefix for everything
- always check it enabled before touching the rest
- rm duplicated code in parse_server_hello()
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 51dd4e1..bc38b8b 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -227,8 +227,8 @@
 #endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 #if defined(MBEDTLS_SSL__ECP_RESTARTABLE)
-    int ec_restart_enabled;             /*!< Handshake supports EC restart? */
-    mbedtls_ecdsa_restart_ctx rs_ctx;   /*!< ECDSA restart context          */
+    int ecrs_enabled;                   /*!< Handshake supports EC restart? */
+    mbedtls_ecdsa_restart_ctx ecrs_ctx; /*!< ECDSA restart context          */
     enum {
         ssl_ecrs_init = 0,              /*!< just getting started           */
         ssl_ecrs_ske_read,              /*!< ServerKeyExchange was read     */