This list (TAILQ actually) is used to suspend / shutdown / detach child
devices in the order opposite to the order in which they attached.
Also, this list is used to resume the child device in their attachment
order.
This is a followup to rS333268 that used device order for those tasks.
Devices generally attach in their order, but not always, because, for
example, of bus passes. So, now we record the real attachment order
instead of approximating it. Note that the order is per bus, not
global.