aix: improve AIX compatibility for Node.js use

As we work through the changes to Node required to support
PowerPC platforms, we have had to introduce a changes to the
libuv component to support the AIX operating system.

These are the initial enabling changes required.
> The second is specifically identifying 0,1,2 files as stdio pipes - then reporting them as UV_NAMED_PIPE - which is what you're pointing out.
> 
> Is there a return value that would be more appropriate for stdio pipes?
> 
> In testing Node - this seemed to do the right thing for the tests. I can imagine scenarios where someone has closed stdout and then re-allocated that special numbered handle to point at something else. This would of course break the logic here, but we're already doing a best guess work-around. AIX simply doesn't support what we need.

Well, if AIX doesn't support it then we need to return UV_HANDLE_UNKNOWN. I'd rather return no result than a misleading one.