POSIX AIO is great, but it lacks vectored I/O functions. This commit fixes
that shortcoming by adding aio_writev and aio_readv. They aren't part of
the standard, but they're an obvious extension. They work just like their
synchronous equivalents writev and readv.
It isn't yet possible to use vectored aiocbs with lio_listio, but that could
be added in the future.