onlyvalue aggregate (was: First Aggregate Funtion?)

Hi,

Here's a patch for the aggregate function outlined by Corey Huinker in
CADkLM=foA_oC_Ri23F9PbfLnfwXFbC3Lt8bBzRu3=CB77G9_qw@mail.gmail.com .  I
called it "onlyvalue", which is a horrible name, but I have nothing
better to offer.  (Corey called it "only", but that doesn't really work
since ONLY is a fully reserved keyword.)

I'll add this to September's commit fest, but if you want to bash me or
the patch in the meanwhile, go ahead.


.m