reboot(9): Shutdown system by signaling init(8) from kernel where appropriate

Some ports watch temperature in MD and try to shutdown system, when overheated,
by calling directly (re)boot(9).  Change this to signal init(8) from kernel,
as acpi(4) and acpitz(4) do, which has been more tested.  This also helps to
clarify design that ``clean shutdown must be always started by init(8)''.

Note that SIGUSR2 causes init(8) to call reboot(2) with RB_POWERDOWN, as
acpi(4) etc. already does.  It is reasonable, pointed out by kettenis@,
considering system is overheated and immediate reboot may be dangerous.

OK deraadt@ miod@

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