macos: add anchors to synthetic.conf grep pattern

macos: add anchors to synthetic.conf grep pattern

The grep pattern that checks if /etc/synthetic.conf already has an entry
for afs is intended to check if this file holds a single column entry
named afs. Unfortunately, the current version does not completely
enforce this restriction. To fix this problem, add anchors to the grep
pattern in question.

Reviewed-on: https://gerrit.openafs.org/14062
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Yadavendra Yadav <yadayada@in.ibm.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 6d6a28720f4eae4652f2628fdfcc30983916f39d)

Change-Id: Iea837157a9eb5c066d577c705c445e10e244757d
Reviewed-on: https://gerrit.openafs.org/14068
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>