auth: Restructure userok

auth: Restructure userok

Restructure the userok logic in auth to split it into smaller functions,
and abstract out common code. This will make it easier to add support
for other security layers in future commits.

Change-Id: I64f6e053ad1a6a2054630a400f7499d805bb9838
Reviewed-on: http://gerrit.openafs.org/2752
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>