ACCELERATION_SPLIT: get started!
ACCELERATION_SPLIT: make sure the queue has always enough free slots.
ACCELERATION_SPLIT: for a start, add the most simple case.

This matches ACCELERATION_RAMPING and works?
dda_queue.c: fix queue_full().

Contribution by @phord, he wrote: I think these ternary tests will
always be true because mb_tail and mb_head are unsigned.