is a 32-bit socklen_t, do_get3() passes the kernel to access the wrong
32-bit half on big-endian LP64 machines when simply casting the 64-bit
size_t optlen to a socklen_t pointer.
While at it and given that the intention of do_get3() apparently is to
hide/wrap the fact that socket options are used for communication with
ipfw(4), change the optlen parameter of do_set3() to be of type size_t
and as such more appropriate than uintptr_t, too.
MFC after: 3 days
35fedb74e5