OPENAFS-SA-2018-002 ptserver: prevent PR_ListEntries information leak

OPENAFS-SA-2018-002 ptserver: prevent PR_ListEntries information leak

PR_ListEntries (pts listentries) does not properly initialize its output
buffers.  This leaks ptserver memory over the wire:

struct prlistentries
- up to 62 bytes for each entry name (PR_MAXNAMELEN 64 - 'a\0')

Initialize the buffer, and remove the now redundant memset for the
reserved fields.

(cherry picked from commit 9d1aeb5d761581a35bef2042e9116b96e9ae3bf5)

Change-Id: I679c205502941891cbb34f10e648a6f9d83c3c60