SCHED_4BSD scheduling quantum mechanism appears to be broken for some time.
With switchticks variable being reset each time thread preempted (that is
done regularly by interrupt threads) scheduling quantum may never expire.
It was not noticed in time because several other factors still regularly
trigger context switches.

Handle the problem by replacing that mechanism with its equivalent from
SCHED_ULE called time slice. It is effectively the same, just measured in
context of stathz instead of hz. Some unification is probably not bad.