OPENAFS-SA-2018-002 volser: prevent AFSVolMonitor information leak

OPENAFS-SA-2018-002 volser: prevent AFSVolMonitor information leak

AFSVolMonitor (vos status) does not properly initialize its output
buffers.  This leaks information from volserver memory:

struct transDebugInfo
- up to 29 bytes in member lastProcName (30-'\0')
- 16 bytes in members readNext, tranmitNext, lastSendTime,
  lastReceiveTime

Initialize the buffers.  This must be done on a per-buffer basis inside
the loop, since realloc is used to expand the storage if needed,
and there is not a standard realloc API to zero the newly allocated storage.

[kaduk@mit.edu: update commit message]

(cherry picked from commit 26924fd508b21bb6145e77dc31b6cd0923193b72)

Change-Id: Id10aa1f4d0b8694f6d85468d743c2fc2a8102339