[0.15.16] Modded achievements copy to vanilla on restart

[Kirona]

Achievements earned while playing Factorio with mods don't trigger the Steam achievements, which is expected.

However, if you then load the game without mods - say, to play a vanilla map, or to play with friends, etc. - all of the achievements you got while playing a modded game (saved in achievements-modded.dat) are immediately unlocked in Steam. They are also migrated over to the non-modded achievements list (achievements.dat).