Refactor poll_oneoff on Unix for easier implementation on Windows.

[marmistrz]

I'd like to merge it before we land any kind of socket-related code. The absence of non-regular files on Windows makes it much easier to implement poll_oneoff on Windows.

Tasks:

• Refactor the code to share common parts across platforms
• Actually implement poll_oneoff on Windows.

[kubkon]

@marmistrz could you put the refactoring bit on hold until #137 lands? I envision some major changes to come out of that and wouldn't want you to spend too much time on refactoring potentially dead code.

[marmistrz]

@kubkon yes. I've just realized that my Windows implementation is kind of blocked by #137 too.

[kubkon]

@kubkon yes. I've just realized that my Windows implementation is kind of blocked by #137 too.

Yeah, my apologies for that! I'll try to merge #137 ASAP so as not to block you! :-)

[marmistrz]

Closing in favor of #137.