nixos/ssmtp: declare all option renames manually

While renaming `networking.defaultMailServer` directly to
`services.ssmtp` is shorter and probably clearer, it causes eval errors
due to the second rename (directDelivery -> enable) when using e.g. `lib.mkForce`.

For instance,

``` nix
{ lib, ... }: {
  networking.defaultMailServer = {
    hostName = "localhost";
    directDelivery = lib.mkForce true;
    domain = "example.org";
  };
}
```

would break with the following (rather confusing) error:

```
error: The option value `services.ssmtp.enable' in `/home/ma27/Projects/nixpkgs/nixos/modules/programs/ssmtp.nix' is not of type `boolean'.
(use '--show-trace' to show detailed location information)
```