macos: add entry for afs into synthetic.conf

macos: add entry for afs into synthetic.conf

The root mount point is read-only as of macOS 10.15. As a result, /afs
cannot be created at this location. To workaround this restriction,
macOS 10.15 provides an alternative way to create mount points at the
root. To make it possible, an entry for the mount point in question must
be added to /etc/synthetic.conf. The synthetic entities described in
this file are not physically present on the disk. Instead, they are
synthesized by the kernel during system boot.

This commit adds an entry for afs into the file mentioned above. Knowing
that this change only takes effect after reboot, also provide directions
to the user during the installation process.

Reviewed-on: https://gerrit.openafs.org/13928
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Yadavendra Yadav <yadayada@in.ibm.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit daf6616aab6732d6b417c15f6f401731ef8e44b5)

Change-Id: If990608d968061ac8ab0391dbd83d1c6a87d32a6
Reviewed-on: https://gerrit.openafs.org/14037
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>