We weren't terribly consistent about whether to call Apple's OS "OS X"
or "Mac OS X", and the former is probably confusing to people who aren't
Apple users.  Now that Apple has rebranded it "macOS", follow their lead
to establish a consistent naming pattern.  Also, avoid the use of the
ancient project name "Darwin", except as the port code name which does not
seem desirable to change.  (In short, this patch touches documentation and
comments, but no actual code.)

I didn't touch contrib/start-scripts/osx/, either.  I suspect those are
obsolete and due for a rewrite, anyway.

I dithered about whether to apply this edit to old release notes, but
those were responsible for quite a lot of the inconsistencies, so I ended
up changing them too.  Anyway, Apple's being ahistorical about this,
so why shouldn't we be?