ioprio-util: add macro for default ioprio settings

ioprio-util: add macro for default ioprio settings

IOPRIO_CLASS_NONE with any priority value actually is an alias for
IOPRIO_CLASS_BE with priority value 4 – which is the default ioprio for
all processes.

We got this right at one place, but wrong at three others (where we
assumed the default value was 0, not 4). Let's add a
macro that encodes this properly, and use it everywhere.