grab-bag of SB-SPROF improvements.

grab-bag of SB-SPROF improvements.

 (1) Thread distribution handler must not check *SAMPLING*, as it
     is thread-local.

 (2) Return the values from the final round when looping.

 (3) Better warning on no sampling process: list the reasons users can
     do something about.

 (4) Update WITH-PROFILING docstring to reflect reality.

 (5) When printing *SAMPLES*, don't print the array.