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.

Change-Id: I29d70c7e4dd567b8b046037f29f71911b8a0593f