macos: packaging support for MacOS X 10.13 This commit introduces the new set of changes / files required to successfully create the dmg installer on OS X 10.13 "High Sierra". Change-Id: Id9da3cf959627a13d8cfd1d1d7412820e46ad63e Reviewed-on: https://gerrit.openafs.org/12742 Tested-by: BuildBot <buildbot@rampaginggeek.com> Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net> Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>