Add SSL presets.

No need to use a separate profile as in X.509, everything we need is already
in ssl_config. Just load appropriate values.
diff --git a/programs/ssl/dtls_client.c b/programs/ssl/dtls_client.c
index 4052975..cc0d051 100644
--- a/programs/ssl/dtls_client.c
+++ b/programs/ssl/dtls_client.c
@@ -163,7 +163,8 @@
 
     if( ( ret = mbedtls_ssl_config_defaults( &conf,
                    MBEDTLS_SSL_IS_CLIENT,
-                   MBEDTLS_SSL_TRANSPORT_DATAGRAM ) ) != 0 )
+                   MBEDTLS_SSL_TRANSPORT_DATAGRAM,
+                   MBEDTLS_SSL_PRESET_DEFAULT ) ) != 0 )
     {
         mbedtls_printf( " failed\n  ! mbedtls_ssl_config_defaults returned %d\n\n", ret );
         goto exit;