nixos/dhparams: Set default bit size to 2048

@Ekleog writes in https://github.com/NixOS/nixpkgs/pull/39526

:

> I think a default of 4096 is maybe too much? See certbot/certbot#4973;
> Let's Encrypt supposedly know what they are doing and use a
> pre-generated 2048-bit DH params (and using the same DH params as
> others is quite bad, even compared to lower bit size, if I correctly
> remember the attacks available -- because it increases by as much the
> value of breaking the group).

> Basically I don't have anything personal against 4096, but fear it may
> re-start the arms race: people like having "more security" than their
> distributions, and having NixOS already having more security than is
> actually useful (I personally don't know whether a real-size quantum
> computer will come before or after our being able to break 2048-bit
> keys, let alone 3072-bit ones -- see wikipedia for some numbers).

> So basically, I'd have set it to 3072 in order to both decrease build
> time and avoid having people setting it to 8192 and complaining about
> how slow things are, but that's just my opinion. :)

While he suggests is 3072 I'm using 2048 now, because it's the default
of "openssl dhparam". If users want to have a higher value, they can
still change it.

Signed-off-by: aszlig <aszlig@nix.build>