Fix eio_unix_is_blocking

[patricoferris]

I think the stubs were previously incorrect as the == had higher precedence than the & so it was always returning false, I double checked with a small program:

external eio_is_blocking : Unix.file_descr -> bool = "eio_unix_is_blocking"

let () =
  let r, _ = Unix.pipe () in
  print_endline (string_of_bool @@ eio_is_blocking r);
  Unix.set_nonblock r;
  print_endline (string_of_bool @@ eio_is_blocking r)

Which printed false false. You can see the apple C compiler complaining about it here: https://github.com/ocaml-multicore/eio/actions/runs/4870762082/jobs/8686910729.

Afaict old programs using eio_posix shouldn't actually be impacted in terms of correctness as this was more of an optimisation to see if we need to wait for an FD to be readable/writeable.

[talex5]

Oops!

[talex5]

Note: @haesbaert has some ideas on how to improve warnings from C #448 (comment)

[talex5]

Adding a dune-workspace file with the following seems to do the trick:

(lang dune 3.7)
(env (_ (c_flags :standard -Wall -Werror)))