viced: kill HOST_TO_ZERO macro

Move all struct host fields that are zeroed into a new structure
host_to_zero which is then included in struct host as 'z'.

Remove the HOST_TO_ZERO macro entirely as its presence can lead
to the introduction of dangerous errors.

Change-Id: I44afd37413afae4e4357e68cdadf5c28784611c5
Reviewed-on: http://gerrit.openafs.org/11287
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: D Brashear <shadow@your-file-system.com>