Set cold to 1 before executing the DVACT_POWERDOWN handlers when halting or

Set cold to 1 before executing the DVACT_POWERDOWN handlers when halting or
rebooting a machine, like it is done in the hibernate case.

At least some USB host controller drivers rely on this to busy way instead
of sleeping.  Avoid a panic on macppc with an uhci(4) cardbus plugged in.

ok deraadt@, uebayashi@