Change in return value of RPLACA/RPLACD

[JONL at MIT-MC (Jon L White)]

There has never been a serious proposal to do this - RMS
was merely being obstinate when we proposed to change the
return value of SETPLIST.  Likely, SETPLIST will return its
second arg (but I was curious to know who depended upon the
return value at all of SETPLIST).