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/ecdsa.c b/programs/pkey/ecdsa.c
index ecb6c22..b474060 100644
--- a/programs/pkey/ecdsa.c
+++ b/programs/pkey/ecdsa.c
@@ -163,9 +163,9 @@
     mbedtls_printf( "  . Computing message hash..." );
     fflush( stdout );
 
-    if( ( ret = mbedtls_sha256_ext( message, sizeof( message ), hash, 0 ) ) != 0 )
+    if( ( ret = mbedtls_sha256_ret( message, sizeof( message ), hash, 0 ) ) != 0 )
     {
-        mbedtls_printf( " failed\n  ! mbedtls_sha256_ext returned %d\n", ret );
+        mbedtls_printf( " failed\n  ! mbedtls_sha256_ret returned %d\n", ret );
         goto exit;
     }