Add some stuff to prevent connection errors

[57uff3r]

Hi!

We use this library in our application and we send thousands of push notifications every day.
And quite often we received connection errors. I added some socket connection options to prevent connection errors. With this options we didn't get any connection errors for almost two weeks.

[djacobs]

Thanks, @57uff3r

[ExplodingCabbage]

This looks like it probably does something worthwhile and would be good to get merged, but I know @djacobs is pretty conservative about merging anything that he doesn't understand and there's some reasonably complicated shit going on in here.

@57uff3r, if you still remember this stuff, could you summarize the changes here and what they do? What does each of the settings here do and why do you need to only do some of them on non-Darwin OSes? Is that Darwin check some sort of standard, documented recipe that you can cite somebody recommending? If not, what's the rationale behind it; what does it avoid?