openconnect: Add darwin support

The old build was failing on darwin because yubikey needs the PCSC framework dependency. I also upgraded to the latest vpnc-script as recommended on the openconnect homepage (see: https://www.infradead.org/openconnect/platforms.html). Lastly, I moved all dependencies which are not used at runtime from propagatedBuildInputs to buildInputs (Please correct me if that is wrong).