Let rows change their rowsource

(rasqal_row_set_rowsource): Added for general internal use

(rasqal_row_set_weak_rowsource): Added for rowsource_rowsequence so
it does not hold circular references to itself in the rows it
stores in it's sequence.
(rasqal_new_bindings_from_var_values): Hanle NULL values sequence
Add rowsource reference counting to rasqal_row
Allow getting variables from a row

(rasqal_row_get_variable_by_offset): Added

(rasqal_query_results_get_binding_name): Use above instead of
variables table.