[bug#32465] Add iptables service

[Arun Isaac]

> it's not directly an answer to arun's patch (it is great), but I 
> recently came accross firemason 
> (http://www.cs.yale.edu/homes/zhai-ennan/firemason.pdf and 
> https://github.com/BillHallahan/FireMason) and I thought we could 
> implement something similar.

This sounds interesting. I'll read about it, and if I can, try to
implement something similar for Guix.

> Of course, we should provide a mechanism to load files as a fallback, in 
> which case additional rules from services should be ignored.

Yes, I think this iptables service should exist independently as a
fallback. So, I'll finish and push these services before studying
firemason.

> PS: Arun, in your patch for the manual you say: "This is the service 
> type to set up an iptables coniguration". This should be 
> "configuration".

Sure, will fix this typo.