* Using a separate dict for globals/locals for each interpreter means we
still get separation of variables and no persistence where we don't
want it, but removing sub-interpreters means pyside can work as it
uses the PyGILState_ APIs which do not support sub-interpreters.
* We import everything up front then duplicate the __main__ each time we
create a new context so we keep the __main__ pristine and muck up an
individual copy.
* Because sys is now shared, the output redirectors that overwrite
sys.stdout and sys.stderr have a NULL context, and instead they look
up a specific global which contains the actual context pointer.
baldurk