ptserver: Optionally restrict anonymous access to the ptserver

Currently, one could simply query from 0 to 'pts listmax' to determine
all the usernames in a cell.  The -restrict_anonymous option will block
access to almost all of the unauthenticated RPC's.   PR_NameToID is still
open since aklog still needs access to this RPC.  An "attack" against
this RPC would have to scan a much larger key space to determine valid
usernames in a cell.

Change-Id: I7e475bc004f08d28d195c199804befa89f0ceb0c
Reviewed-on: http://gerrit.openafs.org/10951
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Gergely Risko <gergely@risko.hu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>