[AD] yield_timeslice

[Elias Pschernig <elias@xxxxxxxxxx>]

I remember, there was some doubts here about changing the behavior of
yield_timeslice, and similiar thoughts about it are present on
allegro.cc:
http://www.allegro.cc/forums/view_thread.php?_id=338662
And when I asked Peter in #allegro what he thinks, he refused to give a
clear answer. Here the relevant logs: :)

<networm> tjaden: do you also think yield_timeslice shouldn't give up
          CPU time, like it does in 4.1.13 now?
<tjaden> networm, leave me out of this :-)
...
<tjaden> i hate computers; i'm going to be a monk

So now, I'd tend to change back to the old behavior of yield_timeslice,
and provide an al_idle or sleep_timeslice, which would be also used
inside Allegro in place of yield_timeslice. (al_sleep I don't like
anymore since there's already rest.)

-- 
Elias Pschernig <elias@xxxxxxxxxx>