[Distutils] easy_install -f confusion

Jeff Hammel and myself were going over a problem with -f seemingly being ignored in easy_install, and it took us a while to realize that you have to do something like "easy_install -f 'url1 url2 ...'", and you can't do "easy_install -f url1 -f url2".

It's literally been years that I've been unaware of this, and confused by the results. I think easy_install should give an error if you provide multiple -f's (or make it work if you do so).