Finish eio_posix backend

[talex5]

From #448:

• Fix errors from getaddrinfo (see Getaddrinfo raises error on some backends #351).
• Add a systhread pool of workers for eio_posix #607
• Decide whether/how to make this more flexible, so that e.g. most of this can be reused with kqueue systems.
• Ensure we yield on IO to prevent starvation (eio_posix: yield on IO #477).
• Mention eio_posix in the documentation #458

[haesbaert]

Concerning getaddrinfo, I wrote this (didn't want to throw it away): https://github.com/haesbaert/ocaml-getaddrinfo
It's in good shape and I can make an opam release at some point.

Basically bindings that can work in an async way via a systhread or fork (Getaddrinfo.Async.P and Getaddrinfo.Async.T)

[talex5]

Ah, I also have: https://github.com/talex5/eio/commits/getaddrinfo (unfinished, but does avoid license problem).