Use the POSIX select(2) call only as a last resort

The Nx_poll module should be unceremoniously ripped out and replaced by a proper shim over kqueue(2) and epoll(4).

I am embarrassed that this problem remains unaddressed.

Still not fixed in Oni 1.0.12, and I think it won't be fixed. When OCaml 4.04 provides implicit modules, I'm thinking I'll want to refactor the whole of Oni to use them, and I'll be doing that on the sideline branch.