nixos/mincraft-server: refactor

 - allow for options to (added 2 options):
   - agree to eula (eula.txt) true/false will create symlink over
     existing eula.txt to `/nix/store/...`.
   - whitelist users (optional and will symlink over existing
     whitelist.json and create backup)
   - server.properties can be configured with the serverProperties
     option. If there is an existing server.properties it will
     copy it to a server.properties.old to keep the old
     one. server.properties MUST be writable thus symlinking is not
     an option.
  - all ports that are stated in `server.properties` are exposed
    properly in the firewall.

(infinisil) nixos/minecraft-server: Fix, refactor and polish

Adds an option `declarative` (defaulted to false), in order to stay
(mostly) backwards compatible. The only thing that's not backwards
compatible is that you now need to agree to the EULA on evaluation time,
but that's guarded by an assertion and therefore doesn't need a release
note.