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

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

SPR_IDToName does not completely initialize the return array of names,
and thus leaks information from ptserver memory:

- up to 62 bytes per requested id (PR_MAXNAMELEN 64 - 'a\0')

Use calloc to ensure that all memory sent on the wire is initialized,
preventing the information leak.

[kaduk@mit.edu: switch to calloc; update commit message]

(cherry picked from commit 70b0136d552a0077d3fae68f3aebacd985abd522)

Change-Id: I787fc26ecb6fa64b17f8579198793903bc4eb16d