Fix for #4070

Fix for #4070

- Add support for a new configuration option "dns-lookup" to
  control weither DNS lookups shall happen at all
- Find servers from DNS always if the dnslookup option is true. This
  way these servers can be used as fallback even if a server is
  configured. Otherwise DNS lookups do not happen if a server is
  configured in the client configuration.
- Support multiple servers in the configuration file.
- Use a global for tracking the registration tries, because
  otherwise the used server will not be exchanged, because
  the try counter gets reset everytime a new register event
  is yielded
- Minor bugfix: Do not try to compare $res to "0" if $res is not
  defined.

git-svn-id: https://oss.gonicus.de/repositories/gosa/branches/2.6-lhm@16397 594d385d-05f5-0310-b6e9-bd551577e9d8