ddplugin: get_netrc_auth needs to be deprecated and replaced.

[leamas]

The get_netrc_auth() method is not fully compatible with the new authentication plugin scheme since it does not have any information whatsoever about which plugin to use.

Current code uses a global which is not that satisfactory. OTOH, the function is part of the interface which should be kept stable, so it cannot just be changed or removed. This means that while adding new methods is fine and probably what should be done, the current broken interface should be kept for now.

And hence the pesky global.