afs: add a file-level comment to afs_osidnlc.c

afs: add a file-level comment to afs_osidnlc.c

This file doesn't currently do a great job of telling the reader what
it's used for.  Let's give them a hint, especially for the expansion
of "DNLC".

Reviewed-on: https://gerrit.openafs.org/13557
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 3a5ab19fe04058e002bfea90f8b64fab4676de67)

Change-Id: I41c53e5a37cc2da8fa0578a9c5db476530103981
Reviewed-on: https://gerrit.openafs.org/13650
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>