Patch for setdiff.m

[John W. Eaton]

On  1-May-2006, Robert S. Weigel wrote:

| 
| 2006-05-01  Bob Weigel <address@hidden>
| 
|         * scripts/set/setdiff.m: Added a "rows" option and 5 "asserts".

I applied this patch then further modified it so the code is in the
form

  if (byrows)
    new code;
  else
    existing code;
  endif

instead of 

  if (byrows == 0)
    existing code;
  endif
  if (byrows == 1)
    new code;
  endif

I also updated the doc string (based on the second patch you sent).

Thanks,

jwe