setup: Cancel button focus is error-prone

[Thomas Wolff]

In setup, the Cancel button should never be focussed by default.
This seems to happen whenever none of the other buttons is enabled, 
e.g. while downloading mirrors.lst and setup.bz2 files.
During this time, it is likely to accidently cancel setup by just 
hitting Enter when the user actually wants to continue setup (and hits 
Enter a little bit early).
This is "unexpected behaviour" in terms of usability and should be changed.

Kind regards,
Thomas Wolff