(rasqal_query_results_set_boolean): Added internal method
Add support for boolean results from a query result foramt.

rasqal_query_results_get_boolean_func handler func added.

struct rasqal_query_results_format_factory_s gains factory method
get_boolean with type above.

(rasqal_query_results_formatter_get_boolean): Added to call method
get_boolean if not NULL.
(rasqal_query_results_formatter_read): Allow boolean results
Add boolean result format support for SPARQL XML

Refactor XMl parsing code to handle bindings / boolean

(rasqal_sparql_xml_start): Added to init XML parsing

(rasqal_sparql_xml_sax2_start_element_handler): Handle boolean
element start (NOP).

(rasqal_sparql_xml_sax2_characters_handler): Handle boolean element
content - save it.

(rasqal_sparql_xml_sax2_end_element_handler): Handle boolean element
content:
- allow <head> to end with no variables
- check string for 'true' or 'false'.  FIXME: handle case independent?

(rasqal_rowsource_sparql_xml_init): Move most of it to
rasqal_sparql_xml_start()

(rasqal_rowsource_sparql_xml_finish): Move most of it to
rasqal_sparql_xml_free_context()

(rasqal_sparql_xml_init_context): Added, pulled out of
rasqal_query_results_get_rowsource_sparql_xml()

(rasqal_sparql_xml_free_context): Added, pulled out of
rasqal_rowsource_sparql_xml_finish()

(rasqal_rowsource_sparql_xml_get_boolean): Added new factory method
to do all the XML work right here.

(rasqal_query_results_get_rowsource_sparql_xml): Slimmed down calling
rasqal_sparql_xml_init_context() to do most init working.

(rasqal_query_results_sparql_xml_register_factory): Register
get_boolean method.
Support boolean results in manifest code and check for pass/fail.
Support boolean results in check-query and check for pass/fail.