(rasqal_new_group_concat_expression): Unsigned bit flags
Make internal limit, offset and distinct vars be unsigned (-Wconversion)

(rasqal_new_orderby_algebra_node, rasqal_new_slice_algebra_node, ): Take
unsigned vars.

(rasqal_algebra_query_add_slice, rasqal_query_print): Fix checks for <= 0
that now can be just = 0
Cast macros for -Wconversion (work in progress; many to go)
Document -Wconversion disabled
Disable "-Wformat-nonliteral" with GCC pragma for test code