Settings in json

Save/load  map_meta.json (once read .txt for convert)

possible use freeminer.json instead freeminer.conf (if freeminer.json exists)
to start use:
echo {} > freeminer.json
freeminer --config freeminer.json

send mapgen params to client (for correct params in enable_local_map_saving and future things)
Why do we send an empty pos here?