When deregistering a bus, attempt to flush out all outstanding

operations before returning. Point the bus at a dummy cam_sim
structure so that any CCBs will complete immediately with a
CAM_DEV_NOT_THERE status, and ensure that any xpt_schedule() calls
on the bus's devices will immediately call the peripheral's
periph_start() routine. Also repeat the async messages because
devices that were part of the way through being probed may appear
after the original AC_LOST_DEVICE was sent, and would otherwise
never go away.

These changes make it possible to deregister a bus and free the SIM
at most stages during bus probing without the usual crashes in
camisr(). In particular, plugging in a umass device and then
unplugging it as soon as the first probe messages appeared would
almost always result in a crash. Now the device just goes away with
a few CAM errors and all references to the CAM bus, target and
device are dropped correctly.

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