Minor buffer.c style cleanups
Merge pull request #998 from war1025/dev/buffer_cleanup

Minor buffer.c style cleanups
eval: re-remove USE_CR #1002

It was already removed in 01ca460 and I erroneously introduced it again in
PR #978.
Units: Move NULL, OK, and FAIL to helpers.moon. #982
eval: re-add has('mac') and has('macunix') #976

They were removed but they can be handy to separate some things to do at
startup in a .vimrc for example.
documentation: update documentation for :mode

see #588