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/library/ssl_tls.c b/library/ssl_tls.c
index 50222c3..90331ef 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5473,7 +5473,7 @@
 #endif
 
 #if defined(MBEDTLS_SSL__ECP_RESTARTABLE)
-    mbedtls_ecdsa_restart_init( &handshake->rs_ctx );
+    mbedtls_ecdsa_restart_init( &handshake->ecrs_ctx );
 #endif
 
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
@@ -7309,7 +7309,7 @@
 #endif /* MBEDTLS_X509_CRT_PARSE_C && MBEDTLS_SSL_SERVER_NAME_INDICATION */
 
 #if defined(MBEDTLS_SSL__ECP_RESTARTABLE)
-    mbedtls_ecdsa_restart_free( &handshake->rs_ctx );
+    mbedtls_ecdsa_restart_free( &handshake->ecrs_ctx );
 #endif
 
 #if defined(MBEDTLS_SSL_PROTO_DTLS)