[Yum] support downgrading off packages

[seth vidal]

> What would also be useful would be some way to display packages that 
> have a version that is not the current version.  This would be useful 
> for detecting packages that came from somewhere else.  "yum list extras" 
> only shows packages that aren't listed in any repo.  It would also be 
> useful for finding downgrades with development system.

doesn't yum list updates get you that information?

-sv