Libevent callbacks should take sockets as evutil_socket_t, not int

On 64-bit Windows, SOCKET of type UINT_PTR, which is 64-bit, whereas int is only 32-bit. To solve this, Libevent 2 defines a type "evutil_socket_t" which is SOCKET on windows and int elsewhere. We should revise all of our libevent callbacks that receive sockets so that they receive evutil_socket_t instead, when libevent2 is present. If we are built against libevent 1.4, however, we should just #define evutil_socket_t to int.