views: search overrides for user and group requests
    
    If the name or the POSIX ID of a user or a group is overridden the
    search request for those objects have to check the overide objects first
    before looking up the original objects.
    
    This patch adds a new request for the IPA sub-domain users which checks
    the overrides first if
    - SSSD is running in ipa-server-mode and a name or a POSIX ID is
      searched, since we do not override the SIDs we can skip the search in
      the override tree here
    - if the responder indicates it has not found the corresponding object
      in the cache and the input might be an override name or ID and not the
      original one of an object.
    
    If an override object was found the SID is extracted from the anchor
    attribute and the original object is search by its SID. If no override
    object was found the original object is search with the original input
    and finally it is checked if an override object exits for the found
    object.
    
    Relates to https://fedorahosted.org/sssd/ticket/2375
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>