sched: Optimize the case of zero timeouts.

sched: Optimize the case of zero timeouts.

If a pre_select method calls sched_min_delay() there is no
point in calling the pre_select hooks of the other tasks since
these can not decrease the timeout any further, and adding file
descriptors to the fd sets makes no sense either. So we may
break out early in sched_preselect() in this case.

If the timeout is zero we may even omit the entire select call as
well as the subsequent gettimeofday() since select() will return
immediately anyway. This patch teaches sched_postselect() to do so
which saves at least two system calls (plus locking in case of
para_server) in a rather hot path.