Extended the GFS search facility. New member of bend_search_rr

Extended the GFS search facility. New member of bend_search_rr
'estimated_hit_count' signals that hits is known to be an estmate
(higher or lower than real hit count). New member 'partial_resultset'
signals that the search was partial (hit count is lower or equal than
real hit count). For Z39.50, the information is stored in resultSetStatus
member of SearchResponse APDU. For SRU, the information is signalled
via SRU diagnostic 59: "Result set created with valid partial results
available".