(gamegrid-add-score-with-update-game-score-1): New function, factored out of
`gamegriod-add-score-with-update-game-score'.
(gamegrid-add-score-with-update-game-score): Use it. Fall back on
`gamegrid-add-score-insecure' if the requested file does not exist in
$(gamedir), in case the user has installed a game on his own.
(gamegrid-add-score-insecure): Accept optional directory argument.