Zebra uses string attributes for indexing internally. Using set+numeric

Zebra uses string attributes for indexing internally. Using set+numeric
use attribute can still be used. But that is a search-only conversion which
inspects .att-set files as indicated using attset-directives in zebra.cfg.
attset references are no longer required, but when used they deserve
as "check" for that the index names used are also present in .att.