main: replace SETRES with target_unmark/mark

Replace SETRES occourences in modules with target_unmark and
target_mark. This way modules don't access result structure directly.
http: use target_unmark before sending probe

xping uses a result buffer of NUM (default 300). Once more that
300 probes have been sent, it will be necessary to clear the result
slot before sending a probe. Otherwise timed out results will display
the old result for that slot.