Apple changed the spelling of its operating system again, to "macOS",
effective with macOS 10.12 Sierra (2016-09-20). Change Emacs
documentation and comments to match this. Stick with older OS
spellings ("OS X", "Mac OS X") when talking about older releases where
the older names are more correct.
* nextstep/README: Move historical information to new file ../etc/NEXTSTEP.
* nextstep/AUTHORS: Merge into ./etc/NEXTSTEP and remove file.
* etc/NEXTSTEP: New file, extracted from ../nextstep/{AUTHORS,README}.
* make-dist: No more nextstep/AUTHORS.