- to read winners from the msgbase, v3.17c (with MsgBase.get_index()) is req'd
* Calculate the difficulty level, no need to store ("level") in winners list
* The winners list object changed a bit ("user"->"name") - you may need to
  delete your data/minesweeper.jsonl, if you have one already
* The default difficulty level (1-5) may now be specified in the [minesweeper]
  section of your ctrl/modopts.ini file, if you don't want the user to be
  prompted. It can also be passed as a cmdline argument, as before.
* Cosmetic improvements (?):
- border around the game board
- both covered and uncovered empty cells are more easily discernable now