[vm/dart:io/fuchsia] Improve fdio_pipe_half signature, step 2.
[vm/dart:io/fuchsia] Improve fdio_pipe_half signature, step 2.

The return value on fdio_pipe_half conflated two things: the error code
on failure (as a zx_status_t) or a file descriptor on success. This
technically worked, because they're both ints, the error code was always
negative, and the file descriptor always positive. However, the stated
return type of zx_status_t was misleading. This changes the signature
such that it always returns an actual zx_status_t, and the file
descriptor is returned through a pointer argument.

Also remove the last argument, since it was always given the same value.

This needs to be done as a soft transition because it's called from the
Dart runtime. The steps are as follows:
1. Add fdio_pipe_half2 with the new signature.
2. Update the Dart runtime to call fdio_pipe_half2.
3. Change fdio_pipe_half to be identical to fdio_pipe_half2.
4. Update the Dart runtime to call the updated fdio_pipe_half.
5. Delete fdio_pipe_half2.

This is step 2.

Change-Id: Ieac841c5f8055f34000851cef364a12b6c0aca2a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/96664
Commit-Queue: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Auto-Submit: Michael Powell <6e880d5938b4dbcdcccde7784a69e2a002dfc041@google.com>
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk