Return empty list on EAI_ errors for getaddrinfo@luv (ocaml-multicore#351)

Unix.getaddrinfo used in u-ring ignores gar_errno and returns an empty list, this
makes the luv backend follow a similar behaviour.

Author: haesbaert

Diff for: lib_eio_luv/eio_luv.ml

@@ -575,10 +575,15 @@ module Low_level = struct
       | _ -> None
     in
     let request = Luv.DNS.Addr_info.Request.make () in
-    await_with_cancel ~request (fun loop -> Luv.DNS.getaddrinfo ~loop ~request ~service ~node ())
-    |> or_raise
-    |> List.filter_map to_eio_sockaddr_t
-
+    match (await_with_cancel ~request
+             (fun loop -> Luv.DNS.getaddrinfo ~loop ~request ~service ~node ()))
+    with
+    | Ok nl -> List.filter_map to_eio_sockaddr_t nl
+    | Error `EAI_ADDRFAMILY | Error `EAI_AGAIN | Error `EAI_BADFLAGS | Error `EAI_BADHINTS
+    | Error `EAI_CANCELED   | Error `EAI_FAIL  | Error `EAI_FAMILY   | Error `EAI_MEMORY
+    | Error `EAI_NODATA     | Error `EAI_NONAME| Error `EAI_OVERFLOW | Error `EAI_PROTOCOL
+    | Error `EAI_SERVICE    | Error `EAI_SOCKTYPE -> []
+    | Error e -> raise (Luv_error e)
 end
 
 open Low_level