Support for Archlinux (and rolling linux releases in general)

I really like the idea of marelle (and that it's written in Prolog!). 

One thing I noticed, when trying to use it with Arch Linux was that it only uses the Codename of the Linux release, which does not work for rolling release distributions like Arch Linux. To solve this issue, I've changed to `linux/1` structure from `linux(Codname)` to `linux(Distroname(Codename))`, e.g. `linux(ubuntu(precise)))` or `linux(arch(unknonw)))`. 
I know, this won't be backwards compatible, but we could probably add an extra predicate to make it backward compatible.

What do you think of this idea?

Note: This patch isn't finished yet, I just wanted to start a discussion about this topic.