tabular_output: move public headers

from tabular_output.h to afsutil_prototypes.h
Like this, tabular_output.h does not have to be included
in any devel-package.

Change-Id: I9e3089fe4a65b2a801c45ba513a8f5dc49ce609b
Reviewed-on: http://gerrit.openafs.org/10327
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>