Add ssort.h defining static rasqal_ssort_r() public domain
(rasqal_sequence_as_sorted): Added if raptor < 2.0.15

Uses inline static rasqal_ssort_r()
(rasqal_query_results_sort): Add user_data arg

If raptor is 2.0.15 or newer, uses raptor_sequence_sort_r otherwise
uses internal rasqal_sequence_as_sorted() to alter the result
sequence directly.

Update callers