Change callback prototype for *_map and *_reduce functions

- remove offset parameter
  for gds_inline_slist_map, gds_inline_dlist_map, gds_slist_map,
  gds_dlist_map, gds_slist_reduce, gds_dlist_reduce
- return value replaces original data
  for gds_slist_map, gds_dlist_map