Consolidate warnings and interval scheduling in doWarning().
Don't let the reboot time drift because of delays on the RunQueue.. each
warning is scheduled based on absolute time from _nextReboot.
doWarning() now takes two args so any subclasses that had overridden
it (unlikley beyond PX) will break...
PendingShutdownObservers will now get the actual milliseconds until reboot.
Basically: after all this, things should essentially work the same except
now there is a protected three-arg version of scheduleReboot that lets you
schedule an exact time, bypassing the normal round-up to the next highest warning time.
Ray J. Greenwell
de3ca2b2ef