MEDIUM: thread: change thread_resolve_group_mask() to return group-local values

MEDIUM: thread: change thread_resolve_group_mask() to return group-local values

It used to turn group+local to global but now we're doing the exact
opposite as we want to stick to group-local masks. This means that
"thread 3-4" might very well emit what "thread 2/1-2" used to emit
till now for 2 groups and 4 threads. This is needed because we'll
have to support group-local thread masks in receivers.

However the rest of the code (receivers) is not ready yet for this,
so using this code with more than one thread group will definitely
break some bindings.