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.
Are you suggesting removing the ```#if defined (_AIX)``` entirely? That would break some Node tests.

This code is assuming that AF_UNIX pipes are named pipes, and doing a special case for 0,1,2 - otherwise it is still returning UV_UNKNOWN_HANDLE.