Added 'run_setup()' to allow outsiders to run a setup script under

fairly tight control, and the '_setup_stop_after' and '_setup_distribution'
globals to provide the tight control.

This isn't entirely reliable yet: it dies horribly with a NameError on the
example PIL setup script in examples/pil_setup.py (at least with Python
1.5.2; untested with current Python).  There's some strangeness going
on with execfile(), but I don't understand it and don't have time
to track it down right now.