[racket] #<undefined> and backward compatibility

I don't think this change would affect any code I've written.

There are a few parts of one large system on which I consult that 
*might* be affected, but probably not.  (Parts written a long time ago 
by unknown superintelligent ancient astronauts, who I suppose might use 
undefined values, or knowingly let them happen incidentally.  And parts 
later worked on by mere humans who wouldn't be aware when they were 
creating situations in which there can be undefined values, especially 
if the undefined values are never actually used.)

Neil V.