pts: Specifically check for group id 0

pts: Specifically check for group id 0

For consistency with the code checking user ids in createuser, check
for a specified group id of 0 specifically and give a slightly
different error message for it.

Reviewed-on: http://gerrit.openafs.org/2994
Reviewed-by: Phillip Moore <w.phillip.moore@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 5e9676144bc3d9b186495a06d14581a4fc97beff)
Change-Id: I37434f1ad44ed1fad41cf454291c91e0b046ec80
Reviewed-on: http://gerrit.openafs.org/3109
Tested-by: Derrick Brashear <shadow@dementia.org>