Add the CellServDB pathname to the afsconf_dir

Add the CellServDB pathname to the afsconf_dir

The determination of the CellServDB pathname is platform-dependent.
However, error reporting in the current code base assumes the CellServDB
location is platform-independent.

Add the pathname of the CellServDB file to the configuration directory
structure and set the new cellservDB member when opening the
configuration. Use this value when checking if the CellServDB has
changed and update the callers to use the cellservDB member when
reporting errors about the CellServDB file.

Change-Id: I5a3393fb9d4ae3c637d5a0d773598115314bfe1c
Reviewed-on: https://gerrit.openafs.org/13408
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>