kernel: new DEBUG_RACE option. try to provoke race conditions between processes.

kernel: new DEBUG_RACE option. try to provoke race conditions between processes.

it does this by
  - making all processes interruptible by running out of quantum
  - giving all processes a single tick of quantum
  - picking a random runnable process instead of in order, and
    from a single pool of runnable processes (no priorities)

This together with very high HZ values currently provokes some race conditions
seen earlier only when running with SMP.