BibSort: initial release

• Create all the necessary infrastructure for the BibSort module
• Bibsort_engine will manage the creation and update of the sorting buckets
• Add new config variable CFG_BIBSORT_BUCKETS for the number of buckets that should be created; if set to 0 it means that bibsort is inactive
• Currently all the configuration (adding/deleting/modifying) of sorting methods is done via the config file; after each modification the config file needs to be loaded in the database
• There has been no major change in the search_engine API; the dispatching of sorting functions is done in the sort_record function
• The bibsort buckets are cached in the search_engine for fast retrieval and sorting of records
• Old style sorting (sort_records_bibxxx) will stil be used in cases were no approprate bibsort method has been detected
• Added support for default method, latest first: data read from bibrec table
• Sort_records_bibsort and sort_records_bibxxx have the same function signature with respect to 'rg'
• 'rg' argument displatched also to sort_records_bibxxx, meaning that also the list of records returned by this function is cut after rg+jrec
• Add documentation