Clean up ipl handling: be sure to always return valid levels in getipl(),

and check input level in setipl() if DIAGNOSTIC.

Doing this pointed out an horrible bug where exception frames would refer
to the saved ipl at the time the exception was processed, but would not
save it before, and the joys of stack values would make it a correct value
90% of the time. Of course, bad things could happen when restoring the
``saved'' ipl...

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