`WSACloseEvent', `WSACreateEvent', `WSAGetOverlappedResult',
`WSARecv', `WSARecvFrom', `WSASend', `WSASendTo' and `WSASetEvent',
`WSAWaitForMultipleEvents'.
* net.cc: Define wsock_evt.
(wsock_event): New class.
(cygwin_sendto): Use overlapped socket io if available.
(cygwin_recvfrom): Ditto.
(cygwin_recv): Ditto.
(cygwin_send): Ditto.
* security.cc (subauth): Set Win32 error to 0 to safely ask for the
error code of dynamically loaded function `LsaRegisterLogonProcess'.