illumos gate

As we try to get upstream patches into more and more software, it would help a lot to have a globally visible macro that identifies us as being illumos rather than conflating us with Solaris or any of our other UNIX heritage. While I appreciate that feature detection is always better, effectively every other operating system has a macro that people can use for simple #ifdef checks and our lack of one merely creates friction that need not exist.

I would propose that we define __illumos__ to be 1.