Improve KPAD function signatures, and small updates to VPAD functions

This is a number of changes focused primarily on the KPAD interface, but includes some changes for VPAD when the changes also applied to them. This does result in some amount of API compatibility breaks, but I think the improved consistency is worth it.

The first change is simply adding a function signature for KPADShutdown(). I'm not sure why this wasn't present, considering that KPADInit() is already there, but adding this is a simple, non-breaking but important addition.

The second change is adding more error codes for both KPAD and VPAD, as well as changing the existing error enum for KPAD to be more consistent with how VPAD labels the errors. I took a look at the disassembly of both KPADReadEx and VPADRead to figure out what all the possible error codes are and make educated guesses as to what the errors indicate. It definitely appears that they share the same error codes, as they both result in error codes in similar locations in their respective read functions, and VPAD has a gap in the possible error codes (-3 is never returned). With that in mind, I renamed the existing KPADError and it's one existing value to match the error enum for VPAD. This, along with using the enum type in the function signature for KPADReadEx, are API breaks, but I think this is still worth it due to how minimal the existing KPAD error enum was previously.

Lastly, I changed the return types for KPADRead, KPADReadEx, and VPADRead from int32_t to uint32_t, as well as updated the comment explaining VPADRead's return value, since it was incorrect. The return value from these functions is the number of entries actually written to the output buffer. Since the functions have an argument with the length of this buffer, it makes sense to have these types match, but they currently don't. Between making the return type unsigned or the argument type signed, I figured the former made the most sense (as it's a case where I'd use size_t, which is equivalent to uint32_t on the Wii U). This is another API break, but may be harder to justify. If you don't think this change is worth it, I can drop the commit from this PR, but I think it's better to make this change now than to just leave it.

devkitPro / wut

Improve KPAD function signatures, and small updates to VPAD functions #173