Clean up of the multi-processor IO module.
The Multiplex_stdout class has been deleted as this is the wrong way of handling the IO streams and is unused anyway.
The PrependStringIO class has had the getvalue() and truncate() methods removed as these need not be implemented as it is provided by StringIO.
Authored by: bugman 2011-08-19
Parent: [r14397]
Child: [r14399]
