sysdb: add overide lookup calls
    
    sysdb_search_user_override_by_name() and
    sysdb_search_group_override_by_name() search for overrides in the given
    view.
    
    sysdb_add_overrides_to_object() adds the data from the override object
    to the original object and makes them available for further processing.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>