Rebranding OS X as macOS

Apple has decided to rename Mac OS X to "macOS", and apparently is now
retroactively referring to old releases that way too.  I propose that
we should do likewise, ie run around and clean up our various references
to "Mac OS X", "OS X", or "OSX" to uniformly say "macOS".  I think it's
considerably clearer to non-Apple people exactly what that is, plus
there's not so much temptation towards variant spellings.

We also have various uses of the ancient code name "Darwin".  I think
we should change that to "macOS" in user-facing docs, but I'm less
inclined to do so in the code.  In particular, renaming the template
file or trying to change the PORTNAME value seems much too messy,
so I think we should stick to "darwin" as the port name.

Barring objections, I'll make this happen (in HEAD only) sometime soon.
        regards, tom lane