Annoying special cases for Mines.

Annoying special cases for Mines.

Firstly, the `Restart' function now reconstructs an initial game
state from the game description rather than dup_game(states[0]).
This means that Restart in a game of Mines restarts to just _after_
the initial click, so you can resume the puzzle-solving part without
having to remember where you placed that click.

Secondly, the midend now contains a second `private' game desc,
which is guaranteed to actually reconstruct the initial game_state
correctly (which Mines's publicly visible game descs tend not to,
since they describe a state which has already had the first click).
This should make serialising of Mines more sensible.

[originally from svn r6025]