Adapt <sys/pslist.h> to use atomic_load/store_*.

Changes:

- membar_producer();
  *p = v;

    =>

  atomic_store_release(p, v);

  (Effectively like using membar_exit instead of membar_producer,
  which is what we should have been doing all along so that stores by
  the `reader' can't affect earlier loads by the writer, such as
  KASSERT(p->refcnt == 0) in the writer and atomic_inc(&p->refcnt) in
  the reader.)

- p = *pp;
  if (p != NULL) membar_datadep_consumer();

    =>

  p = atomic_load_consume(pp);

  (Only makes a difference on DEC Alpha.  As long as lists generally
  have at least one element, this is not likely to make a big
  difference, and keeps the code simpler and clearer.)

No other functional change intended.  While here, annotate each
synchronizing load and store with its counterpart in a comment.

(riastradh)