add cygwin support

[Berrysoft]

This PR adds cygwin support. It uses the POSIX poll as backend. I tried my best to fix the tests. Some of them are ignored reasonably, while some of them are not - related to the UnixListener and UnixStream. I don't understand them yet.

[Thomasdezeeuw]

Mostly LGTM. For the ignored tests can you add a reason why they are ignored.

[Berrysoft]

@Thomasdezeeuw I enabled the 4 ignored tests, adding some workarounds to complete the handshake:

• For UnixListener, it should call accept to unblock the client thread. Fortunately there is always an accept call in the real usages, I think.
• For UnixStream, it should poll after connect to unblock the server thread. I think it should be true in the real usage, where the async runtime always poll after connect.

[Berrysoft]

CI is failing but it's not my problem. The master branch also fails.

[Berrysoft]

I proposed a fix for CI in #1883 .

[Thomasdezeeuw]

Thanks @Berrysoft