setup: make ez_setup.py work to upgrade setuptools even if there is already a setupto...

setup: make ez_setup.py work to upgrade setuptools even if there is already a setuptools installed which is too old

This works only if setup.py is invoked as "./setup.py install" (or
"python ./setup.py install" or whatever).  It doesn't work if it is invoked by
easy_install.  On the other hand, I don't know why easy_install would execute
ez_setup.py anyway -- I thought that it didn't execute the setup.py scripts.
See this mailing list thread for details:

http://mail.python.org/pipermail/distutils-sig/2007-October/008339.html