LDAP: support for multiple LDAP/AD servers

Implements support for multiple LDAP/AD servers, i.e.
- parallel use of n configured LDAP/AD servers
- configuration of a replica LDAP/AD server as backup, including possibility to override main LDAP/AD server
- extended settings interface that allows to create, modify, delete, enable and disable of single LDAP/AD server configurations

Plus some necessary changes under the hood.

Reviewers wanted, e.g. @karlitschek @danimo @xtruthx @MTGap or others