dda.c: more notes on rampup_steps calculation.
According to avr-libc documentation, ISR() handles SREG its self.
timer.c: the concept of a "new move" was abandoned long time ago.

This doesn't matter much, as the timer overflows 300 times/second
worst case, so the very first step of a series of moves is
delayed never more than 30 milliseconds. Hardly recognizeable
by a human.

Saves a nice 40 bytes.
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
140 bytes smaller.