Here's a patch to add unknownin/unknownout support. I also poked around

looking for places that assume UNKNOWN == TEXT. One of those was the
"SET" type in pg_type.h, which was using textin/textout. This one I took
care of in this patch. The other suspicious place was in
string_to_dataum (which is defined in both selfuncs.c and indxpath.c). I
wasn't too sure about those, so I left them be.

Joe Conway