Low: scheduler: calculate group promotion scores correctly

Previously, promotion_score() took a default_score argument, which was set to 0
by one caller and -1 by another caller.

For promotable cloned groups, the result was the sum of calling
promotion_score() for each group member, initializing score to the default and
using this for each child:

if (score == default_score) {
    score = c_score;
} else {
    score += c_score;
}

When the default is -1, and the first <N> members' scores were explicitly set
to -1, they would be ignored, because the next child would hit the = instead of
the +=. However when scores are explicit, they should be taken into account.

Also when the default is -1, a related issue is that if only some members of a
cloned group have promotion scores, members before that without scores would be
ignored, while members after that without scores would be added as -1 each.
However, in this case, all group members without scores should be ignored. In
other words, a group's score should be the sum of only those members that have
explicit scores, or the default value if no member does.

The fix is to always use 0 as the default, but have an output variable for
whether the returned value was explicit or the default, so the caller can
modify its behavior accordingly.