New MD API: rename functions from _ext to _ret

The _ext suffix suggests "new arguments", but the new functions have
the same arguments. Use _ret instead, to convey that the difference is
that the new functions return a value.
diff --git a/programs/pkey/dh_server.c b/programs/pkey/dh_server.c
index f1d3be3..4d8632b 100644
--- a/programs/pkey/dh_server.c
+++ b/programs/pkey/dh_server.c
@@ -217,9 +217,9 @@
     /*
      * 5. Sign the parameters and send them
      */
-    if( ( ret = mbedtls_sha1_ext( buf, n, hash ) ) != 0 )
+    if( ( ret = mbedtls_sha1_ret( buf, n, hash ) ) != 0 )
     {
-        mbedtls_printf( " failed\n  ! mbedtls_sha1_ext returned %d\n\n", ret );
+        mbedtls_printf( " failed\n  ! mbedtls_sha1_ret returned %d\n\n", ret );
         goto exit;
     }