On win64, sockets are of type UINT_PTR; on win32 they're u_int;
elsewhere they're int.  The correct windows way to check a socket for
being set is to compare it with INVALID_SOCKET; elsewhere you see if
it is negative.

On Libevent 2, all callbacks take sockets as evutil_socket_t; we've
been passing them int.

This patch should fix compilation and correctness when built for
64-bit windows.  Fixes bug 3270.