powerd shutdown screen corrupted

[pgf]

on os8, when powerd puts up the large "stop sign" screen, it gets corrupted with vertical dark strips just before the UL screen appears.

observed on both 1.5 and 1.75.

given how fast we shut down, perhaps powerd should display a copy of the UL screen instead of the stop sign. (or, perhaps, nothing at all, but i seem to recall that we need a bridge between the UI and the UL screen, to prevent the console from appearing. but maybe that's not true anymore.)