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.

Reviewed by: kib (earlier version)
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7120