event: support socket sources on Windows

[adierking]

Provide an initial implementation of Windows socket sources using
WSAEventSelect(). This is sufficient for the dispatch_io_net test to
pass on Windows.

[adierking]

cc @compnerd @ktopley-apple

[compnerd]

Should libdispatch invoke WSAStartup somewhere perhaps? Although, I suppose that if the user is creating a socket, they will have initialised winsock and everything should be fine. libdispatch will never construct a socket on its own (unless it is in a test, where we can explicitly call WSAStartup.

[adierking]

Should libdispatch invoke WSAStartup somewhere perhaps? Although, I suppose that if the user is creating a socket, they will have initialised winsock and everything should be fine. libdispatch will never construct a socket on its own (unless it is in a test, where we can explicitly call WSAStartup.

Right, since libdispatch only ever consumes sockets, I think it would be best to let clients initialize WinSock on their own. We don't have any means of knowing whether a program wants to use sockets otherwise, so this might lead to WinSock being unnecessarily initialized for programs that don't need to do any networking.

[compnerd]

@swift-ci please test