ptserver: Limit length on namelist, idlist

ptserver: Limit length on namelist, idlist

namelist and idlist are used as IN parameters to ptserver RPCs that
can be issued by unauthenticated clients. Not having a length limit on
them means anyone can use up a ton of ptserver memory by just issuing
those RPCs with a very large length.

So, put a limit on them. PR_MAXLIST is a constant that already exists,
but is small enough to potentially limit real use, so define a new
OpenAFS-internal value for this purpose.

prlist and prentries are returned from the ptserver to clients, so
also limit them in the same way.

Change-Id: Iaf45639bbae401093354adbfb4daa172fe97ede1
Reviewed-on: http://gerrit.openafs.org/9588
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>