Smarter SuSEFirewall proposal during installation

* Don't propose to enable the firewall if SuSEFirewall is not selected for installation (bnc#881250)
* Avoid wacky proposals when possible (bnc#886554)
We could also do something purely visual in the proposal. But I'm not sure what:

* Changing the string is not an option.
* Including a warning looks overkill.
* I would go for striking the line with the "del" html tag (strike is deprecated), but maybe is too much and I guess it does not work in curses (just guessing).