Rpm packaging with maven

As this change is only activated with the `-Prpm` maven profile, it has no impact as-is. I think we should already merge this partial work, which will increase the chance of getting feedback and will make further changes easier to integrate (yes, I'm lazy and do not like to rebase too often).

As the approach taken is a bit controversial (I'm a much bigger fan of maven than @lookfirst), I'll wait for feedback and will not merge it myself.