slapi-nis: normalize memberUid search filter when searching AD users
    
    memberUid attribute uses IA5 String comparison which is case-sensitive.
    At the same time, uid attribute uses case-insensitive comparison.
    
    When memberUid is constructed for groups from AD, SSSD normalizes names
    to a lower case. slapi-nis records these entries as they produced by SSSD.
    However, the search filter is not modified, thus case-sensitive comparison
    of memberUid attribute may fail match of the original term.
    
    Workaround the issue by low-casing memberUid term in the search filter
    if it includes '@' sign, meaning we are searching on fully-qualified user
    name provided by SSSD.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1130131