busybox

xargs: make -I imply -r

Make -I imply -r (GNU findutils seems to do the same).

Fixes the following bug:

        $ echo -n | xargs -I% echo %
        Segmentation fault

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>