[Yum] support downgrading off packages

[Kristof Vansant]

Is there a way to make to make yum know that it has to downgrade
packages when it upgrades.

Sometimes in rawhide packages get downgraded but yum does not detect
this.

A good example is that the people running fedora test 1 have evolution
1.5.3 installed but should downgrade to 1.4.7 because evolution 2.0
won't make it in fc2. But if you upgrade to rawhide yum won't downgrade
it. You'll have to do it manual.

Lupus (Kristof Vansant Belgium)