Remove static string printfs in utilities (-Wformat)
Enable -Wformat=2 for gcc
Travis CI: Try to exclude some matrix combinations
Use size_t for sv size (-Wconversion)

rasqal_rowsource_sv_context field variables_count changes to size_t
to capture full sv size
Enable -Wconversion
Add -Wswitch-default
Add some default: cases (-Wswitch-default)
Disable -Wswitch-default again
Remove gcc warnings included in -Wall

-Wlogical-not-parentheses -Wsizeof-array-argument -Wbool-compare
Document -Wcast-qual is noisy
(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