Implement Roland Brochards' new acceleration maths clock-based.

His implementation was done on every step and as it turns out,
the very same maths works just fine in the clock interrupt.
Reason for the clock interrupt is: it allows about 3 times
higher step rates.

This strategy is not only substantially faster, but also
a bit smaller.

One funny anecdote: the acceleration initialisation value, C0,
was taken from elsewhere in the code as-is. Still it had to be
adjusted by a factor of sqrt(2) to now(!) match the physics
formulas and to get ramps reasonably matching the prediction
(and my pocket calculator). Apparently the code before
accumulated enough rounding errors to compensate for the
wrong formula.
Fix endstop stop condition for clock-based acceleration.

This required a strategy change, since move_state.n is no longer
decreasing in steps of 1.