Followup commit for #3200

Followup commit for #3200

- Use release dn as search base when checking if a given
  classname is already used by another administrative unit.
- Remove the unittag from the filter that is used to search
  for existing classes. This way every class is found that
  has the same name and it can not happen, that the error
  may not be shown under some circumstances.
- As a side-effect of the previous change, the error might
  also be shown, if the classname is in use in the same
  administrative unit. But this is already handled by a
  standard duplicate check, so to avoid double error message,
  we only check for the classnames, if the previous check didn't
  trigger an error message.

git-svn-id: https://oss.gonicus.de/repositories/gosa/branches/2.6-lhm@14703 594d385d-05f5-0310-b6e9-bd551577e9d8