PM1 power button events were generating 'fake events' to acpibtn PWRB,

PM1 power button events were generating 'fake events' to acpibtn PWRB,
but if there is no such device these were being tossed.  Instead, arrange
for all power button handlers to flag a softc variable which is processed
by the thread; so that one place will know to request a powerdown (psignal
to init)
ok marco mlarkin