Update resolvconf in shell platform

[skeggse]

pritunl-client currently only updates resolvconf/systemd-resolve in graphical mode. This PR contains proof-of-content support for the shell platform.

Note that this duplicates logic from __main__.py, which is suboptimal. I'm not super familiar with the structure of this package, and don't see a good place to migrate the common code to that doesn't introduce a new module.

I'm also not sure what the purpose of these arguments passed to OpenVPN are, and seeing as they aren't passed in the existing shell platform I omitted them. They might need to be re-introduced.