kern.vt.slow_down tunable to make laptop debugging possible

[phk]

Summary

On a laptop with no other console devices than the screen, things scroll of the screen faster than eye or camera can capture it.

The new kern.vt.slow_down tunable slows the console down and makes it update synchronously, so console output continues when timers or interrupts do not.

[imp]

Though why 1000? Might be good to document in the commit message. Ad hoc value that works? Or derived from something else?

[phk]

The loop needs about 1000 iterations before the screen is obviously slower.

I decided to prescale the value by thousand, so that kern.vt.slow_down=1 would do something.