Made overmap file path independent of the game mode.

Fixes #9203 - the overmap is now stored in the same path, independent of the game mode. Previously it was stored as "save/worldname/Tutorial.o.0.0", now it will always be "save/worldname/o.0.0", regardless of the game mode. This ensures that the overmap can be loaded as normal. Btw. this was flawed anyway because the code used the function `special_game_name` which returns a *translated* string. So the path also depended on the language settings.

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kevingranade mentioned in the linked issue:

> I said warn the player that weird things might happen if they continue with a game started in the tutorial.

I have no idea where to add that message, maybe as part of the tutorial messages?