eio_posix: ignore some errors writing to the wake up pipe

The README test failed in CI on macos with:

    # File "README.md", line 1, characters 0-0:
    # /usr/local/bin/git --no-pager diff --no-index --color=always -u _build/default/README.md _build/default/.mdx/README.md.corrected
    # diff --git a/_build/default/README.md b/_build/default/.mdx/README.md.corrected
    # index e710380..e1d7eb2 100644
    # --- a/_build/default/README.md
    # +++ b/_build/default/.mdx/README.md.corrected
    # @@ -1673,7 +1673,7 @@ Joining with the other domain
    #
    #  ```ocaml
    #  # y + Domain.join foreign_domain
    # -- : int = 42
    # +Exception: Unix.Unix_error(Unix.EPIPE, "single_write", "")
    #  ```
    #
    #  we arrive at the answer.

Ignore EPIPE when writing, since it means we've shut down and don't
need a wake-up (the event has presumably already been processed).

Also, ignore EAGAIN and EWOULDBLOCK. If the pipe is full, then a wake-up
is already pending.

Author: talex5

lib_eio_posix/sched.ml

@@ -73,8 +73,15 @@ let wakeup t =
   Rcfd.use t.eventfd
     ~if_closed:ignore       (* Domain has shut down (presumably after handling the event) *)
     (fun fd ->
-       (* This can fail if the pipe is full, but then a wake up is pending anyway. *)
-       ignore (Unix.single_write fd wake_buffer 0 1 : int);
+       try
+         ignore (Unix.single_write fd wake_buffer 0 1 : int)
+       with
+       | Unix.Unix_error ((Unix.EAGAIN | EWOULDBLOCK), _, _) ->
+         (* If the pipe is full then a wake up is pending anyway. *)
+         ()
+       | Unix.Unix_error (Unix.EPIPE, _, _) ->
+         (* We're shutting down; the event has already been processed. *)
+         ()
     )
 
 (* Safe to call from anywhere (other systhreads, domains, signal handlers, GC finalizers) *)