Revert "nixos/sshd: drop mode from auth keys file".

This reverts commit a3331eb8.

See https://github.com/NixOS/nixpkgs/issues/2559#issuecomment-47313334


for a description why this is not a good idea.

I guess it's better to implement a sane way to remove all files in
authorized_keys.d, especially because it is also backwards-compatible.

Reopens #2559.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>