It is not uncommon that a flags type has also the value 0 mapped,
for example to "unknown" or "none".
In that case, we should not return an empty string, but instead
that zero value.
Also, flags actually have an unsigned type. That isn't a real
problem to cast it to a signed int. But be more careful about
it and use unsigned while handling unsigned values and only
cast to int once.
112f09cf4b