macos: packaging support for MacOS X 10.13

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".

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>
(cherry picked from commit e533d0737058940d59d93467c9b4d6d3ec2834e6)

Change-Id: I8932f6a3db6a0572aa36944aa339b888fac94b7d
Reviewed-on: https://gerrit.openafs.org/12826
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>