Write modifiers to sub-SELECT and write VALUES graph patterns
Add indent to rasqal_query_write_sparql_values

(rasqal_query_write_sparql_values): Add indent arg and use it to
indent the values rows

(rasqal_query_write_sparql_graph_pattern,
rasqal_query_write_sparql_20060406): Update callers of above.
(rasqal_query_write_sparql_variables_sequence): Renamed from rasqal_query_write_sparql_select
Make writing sparql seq of var, expr work in same way

(rasqal_query_write_sparql_variables_sequence): Write space between
items only.

(rasqal_query_write_sparql_expression_sequence): Renamed from
rasqal_query_write_expression_sequence. Use count.

Updated callers of above
Use var size for holding raptor_sequence_size values