On shutdown/reboot, send SIGPR as a heads-up to watchdogd that the
system is going down. All file systems are about to be unmounted.
It's just about the last process left running in the system, so we
give it two seconds to complete its work before we continue.
When everything has been unmounted and we're ready to call reboot(),
we send SIGTERM to the registered watchdog, to give ti the chance to
let the WDT reset the system.
Signed-off-by: Joachim Nilsson <troglobit@gmail.com>