Ringing in a new asynchronous I/O API

You can just leave it as a separate library, like libaio is now, and libattr, and libacl, and many others. (Though if it's useful and glibc wants to use it itself, it's not unimaginable that it might find its way in there in time -- but glibc has harsh backward-compatibility constraints that argue in favour of a trial period in an external library in any case, until we know whether the API the library provides works well.)