kernel32 functions restype/argtypes cleanup

Since ctypes caches the kernel32-function look-ups and since the potential exists for the kernel32 functions to be used in other projects, it's better to make use of the data types that are already defined in the ctypes modules.  It's also better, in this regard, to not use data types created by POINTER() since they are more restrictive than just using c_void_p and thus more likely to clash with other projects.  Any changes made to the functions via setting .restype and .argtypes are visible to any code that uses the functions.

These changes make pyreadline easier to use in conjunction with other projects and will help ameliorate issues #21 and #18.  (But colorama needs to stop using POINTER(), etc., as well.)