openssh: Build with Kerberos by default

This can be disabled with the `withKerberos` flag if desired.
Make the relevant assertions lazy,
so that if an overlay is used to set kerberos to null,
a later override can explicitly set `withKerberos` to false.

Don't build with GSSAPI by default;
the patchset is large and a bit hairy,
and it is reasonable to follow upstream who has not merged it
in not enabling it by default.

nixos/modules/misc/nixpkgs.nix

@@ -69,7 +69,6 @@ in
           [ (self: super: {
               openssh = super.openssh.override {
                 hpnSupport = true;
-                withKerberos = true;
                 kerberos = self.libkrb5;
               };
             };