Change how ps_threads and p_thr_link are locked away from using SCHED_LOCK.

Change how ps_threads and p_thr_link are locked away from using SCHED_LOCK.

The per process thread list can be traversed (read) by holding either
the KERNEL_LOCK or the per process ps_mtx (instead of SCHED_LOCK).
Abusing the SCHED_LOCK for this makes it impossible to split up the
scheduler lock into something more fine grained.

Tested by phessler@, ok mpi@