Make sure that ref and changed bits propagate back to the VM layer
whenever pages are unmapped. The old code had several races that could
allow these to become stale.

Approved by:	re (kib)
Rework handling of ofw_quiesce(), making it the responsibility of the
platform modules. Whether to call this function or not is highly machine
dependent: on some systems, it is required, while on others it breaks
everything. Platform modules are in a better position to figure this
out. This is required for POWER hypervisor SCSI to work correctly. There
are no functional changes on Powermac systems.

Approved by:	re (kib)