Improve Windows support (async connections and Unix domain sockets)

[clue]

This PR adds Windows support checking for known error conditions on Windows. This has been tested extensively on some Windows 10 installations, but this repository does not currently run any tests on Windows. I consider this to be out of scope for this ticket, but agree that it makes sense to look into test automation in a follow-up PR. In the meantime, you're invited to run this on your local Windows installation or take my word for granted, so now let's get this shipped! ❤️

For more details about support for Unix domain sockets, see also
https://blogs.msdn.microsoft.com/commandline/2017/12/19/af_unix-comes-to-windows/

Resolves / closes #41
Refs reactphp/child-process#67

[clue]

I've tried to get Travis CI to run the test suite on Windows, but had to give up for now. The test suite works fine locally and I have no reason to believe that the affected code would be subject to any regressions any time soon.

The best test run I could achieve would be https://travis-ci.org/clue/php-socket-raw/jobs/478794485 which only failed due to the missing ext-sockets during installation. I have reason to believe the test would have succeeded otherwise.

I have already spent way to much time on this and I have no interest in chasing this down any further, so I've cherry-picked the changes from the best test run to reactphp/child-process#71 where no special extensions are required and will close this ticket for now. In case anybody feels like looking into the Windows tests, I consider this PR to be a good starting point for a future PR 👍