Add initial simple debian packaging

(my previous pull request got borked while I was trying to rename the branch to be a feature branch.)

Hi, we're going to use easyrsa3 in our company and we require debian/ubuntu packaging to be able to use it. So here is our packaging. I hope it is useful to other people.

Let me know, what needs to be done to get this merged upstream?
Note: We will be available to maintain this. So this is not a maintainance nightmare (things getting added and nobody care for them).