PTM_U: Implemented the GetShellState function.

Uses a new miscellaneous setting for the shell state, default to true.

This function returns whether the 3DS's physical shell casing is open or not.
... I know this was discussed before, but I think you misunderstood my point. Surely enough it should be easy to expose this as an option iff (for whatever reason) someone thinks it should be in the future, however that doesn't mean you should actively plan this in the code now.

Iow: Please indeed make this variable static, and drop the `g_` prefix.