macos: make the OpenAFS client aware of APFS

macos: make the OpenAFS client aware of APFS

Apple has introduced a new file system called APFS. Starting from High
Sierra, APFS replaces Mac OS Extended (HFS+) as the default file system
for solid-state drives and other flash storage devices.

The current OpenAFS client is not aware of APFS. As a result, the
installation of the current client into an APFS volume will panic the
machine.

To fix this problem, make the OpenAFS client aware of APFS.

Change-Id: Ib5ac88b87f348744864f4e33f1f222efbc852d41
Reviewed-on: https://gerrit.openafs.org/12743
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>