Add AFSCONF_NOCELLNAME error code

Add AFSCONF_NOCELLNAME error code

Contrast with AFSCONF_NOCELL, which is for when no cell configuration
information is available at all (i.e., a struct afsconf_dir* was NULL) --
this code is used when there is some cell configuration available, but
that configuration does not include the cell name.

Replace the only existing use of AFSCONF_UNKNOWN with this more-informative
error code, leaving AFSCONF_UNKNOWN free for use in other situations.

Change-Id: I989756a960e5377545af43f8e9414d1f2d6476b4
Reviewed-on: http://gerrit.openafs.org/11628
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>