gcq_merge_tail
gcq_merge_tail(&t->q[Q_CB], &t->q[Q_NEXT_CB]);
gcq_merge_tail(&t->q[Q_CALLBACKS], &t->q[Q_CB]);
gcq_merge_tail(&t->q[Q_CALLBACKS], &t->q[Q_NEXT_CB]);
gcq_merge_tail(&t->q[Q_CALLBACKS], &t->timed);