Mobile broadband with ppp

Here’s the updated connection script that replaces [Pull Request #20](https://github.com/joukewitteveen/netctl/pull/20). I think I’ve fixed the issues you outlined in the discussion there. Setting the `InterfaceRoot` in the script sorts out the issue with enabling it, and most of the `echo` calls have been replaced with `cat` where possible. 

As an aside, during the research for coding this, I discovered that these devices also expose a `wwan` interface which may provide a faster connection than `ppp`, and that it is possible to connect manually to this interface. I didn’t have much success connecting with it in `netctl` though so I’ve moved that code to another branch as a WIP.