Added a TSocketHandle type to winlean.nim and posix.nim

These changes fix a major law in nimrod's usage of Window's sockets - On Windows, socket handles are represented by a pointer to an unsigned long, which is 32 bits on Win32, and 64 bits on Win64. This fixes the problem by declaring a new type in posix.nim and winlean.nim - the TSocketHandle type, and using it for all representations of sockets.