MFC 302860: Fix aio system call wrappers in librt.

• Update aio_return/waitcomplete wrappers for the ssize_t return type.
• Fix the aio_return() wrapper to fail with EINVAL on a pending job. This matches the semantics of the in-kernel system call. Also, aio_return() returns errors via errno, not via the return value.

Approved by: re (gjb)
Sponsored by: Chelsio Communications