A default timeslice of 20ms means a pathological client can ruin up to
two frames per scheduler tick.  And a fifth of a second is just insane.

Pick two different numbers out of the hat.  A 5ms slice means you can
probably keep up with two or three abusive clients, and letting it burst
to 15ms should give you about all the timeslice you need for a
fullscreen game (that's doing server-side rendering for some reason).

If you're running on a system with a 10ms granularity on SIGALRM, then
this effectively changes the intervals to 10ms and 30ms.  Which is still
better, just not as better.

I suspect this is about as good as we can do without actually going
preemptive, which is an entire other nightmare.

Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>