add execute-mode support to the raw reader - allows to directly call commands and read their output.

Note that fdstream.h is from boost and has a separate license:
 * (C) Copyright Nicolai M. Josuttis 2001.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.

src/types.bif

@@ -186,6 +186,7 @@ enum Mode %{
 	MANUAL = 0,
 	REREAD = 1,
 	STREAM = 2,
+	EXECUTE = 3,
 %}
 
 module GLOBAL;