Fix windows client not always exiting

The call to WSAEnumNetworkEvents() was reseting network
events in particular WSAECONNRESET was sometimes being lost
causing the client to not exit.  The WSAEnumNetworkEvents()
was called from fd_is_socket() and has been replace with
getsockopt() to determine if the fd is socket or not.

Broken in commit 4d40a275, v4.3.23