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millisecond it is calibrating. Suggested by jhb@ and bde@. Don't clobber the tsc_freq with the new value since it isn't accurate enough for timecounters and the timecounter system as a whole needs support for changing rates before we do this. Subtract 0.5% from our measurement to account for overhead in DELAY. Note that this interface is for estimating the clockrate and needs to work well at runtime so doing a full calibration including disabling interrupts for a second is not feasible.