yum-presto not on by default

[Kevin Kofler]

I wrote:
> Basically, the safest solution would be to fix xz, then rebuild all noarch
> packages. At least those noarch packages which happened to be built on a
> PPC builder will need rebuilding after xz gets fixed. Please note that
> this probably also affects noarch subpackages.

PS: If we don't care about F11->F12 and Rawhide->F12 deltas, then we don't 
have to rebuild anything, just fix xz before the release.

        Kevin Kofler