Revert "(rasqal_variables_table_get_names): Remove and variable_names field"

This reverts commit 71a1110913c2c57245fc4106586e7910cd83c758.
Copy row values from query results vars table to query.

(rasqal_query_results_update_query_bindings): Renamed from
rasqal_query_results_update_bindings and now reads from query results
vars table and copies row values into query vars table.  Primary for
CONSTRUCT
Use query results vars table to get names

(rasqal_query_results_get_bindings): Use
rasqal_variables_table_get_names() on query results vars table.

(rasqal_row_get_names): Removed since no longer used.
rasqal_query_results manages it's own variables table now

(rasqal_new_query_results2): Deprecated for rasqal_new_query_results
without the vars_table parameter that is now ignored.

(rasqal_query_results_ensure_have_row_internal): Ensure the vars
table is built once from the first row seen.

(rasqal_query_results_get_binding_name): Get binding name from query
results vars table rather than row.

(rasqal_query_execute_with_engine,
rasqal_new_query_results_from_string, rasqal_service_execute,
srxread, srxwrite): Updated all callers of rasqal_new_query_results()
to use rasqal_new_query_results2() with no vars table.