viced: kill CLIENT_TO_ZERO macro

Move all struct client fields that are to be zeroed upon structure
reuse to a new struct client_to_zero.  Include the new structure
within struct client and call memset() on that structure.

Change-Id: I0f83f5f18b41bc0d4f8e1f7f8e04cd5508cbe4e1
Reviewed-on: http://gerrit.openafs.org/11288
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>