libprot: add pr_PrintContEntry function

libprot: add pr_PrintContEntry function

A continuation entry (struct contentry) contains 39 id elements, however
a regular entry (struct prentry) contains only 10 id elements. Attempting
to access more than 10 elements of a regular entry is undefined
behavior.

Add a new function to safely print continuation entries and change
pr_PrintEntry to avoid accessing the entries array out of bounds.

The pr_PrintEntry function is at this time only used by the prdb_check
and ptclient debugging utilities.

Change-Id: Ie836983c8a5970a9495b87d0627ba6c05d117a9b
Reviewed-on: http://gerrit.openafs.org/11750
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>