Add system.unsafeOverrides to quick replace system dependencies

Example usage for the current openssl bug:
```
  system.securityUpdates = with pkgs; [
    ({ old = openssl;
       new = callPackage /path/to/new/openssl {
         fetchurl = fetchurlBoot;
         cryptodevHeaders = linuxPackages.cryptodev.override {
           fetchurl = fetchurlBoot;
           onlyHeaders = true;
         };
       };
    })
  ];
```

I only did nixos-rebuild build because I've a little of fear :P is what I did safe?
@edolstra 

If you intend to merge, please feel free to change it at your discretion, as long as we have a way to replace dependencies for security updates in configuration.nix, I'm fine.