Better handling of previously raised timer interrupts
Better handling of previously raised timer interrupts

My first attempt at handling programs that requested a higher timer
interrupt frequency before tracing started was no robust - it only
handled programs that never again changed the timer interrupt
frequency. Now the script treats that initial time period as
having an interrupt interval of "Unknown ms".

I also updated the comment about the flaws - a lack of handling of
nested calls is the real flaw, but I haven't hit problems with it yet.

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