dda.c: update last_dda also when idle.
dda.c: do acceleration maths only on actual moves.
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 and improves max step rate by several percent.