Fix some -Wextra warnings

Mostly -Wsign-compare warning and -Wold-style-decl changeset in conversion.c and type-u32.c may need attention.

If it's OK maybe adding add_project_arguments('-Wextra', language : 'c') to prevent future warning while developping.

Also I have some stashed change about main args not used if it's worth it.

(Doing this work before trying gcc' -fanalyzer on gimp+babl)