Optimize SELECT * in EXISTS

I read in the SQL standard that SELECT * in EXISTS is not supposed to be
expanded to all columns, but only to an arbitrary literal. This
corresponds to the recommendation in the PostgreSQL documentation to
write EXISTS (SELECT 1 ...) instead. But not even our own tests and
example code use that latter convention consistently, so I think many
users don't know it or observe it either. So implementing that little
optimization for SELECT * seems reasonable.

The attached patch implements the transformation, meaning in EXISTS
(SELECT * FROM ...), the star is replaced by an empty select list
(taking advantage of the support for zero-column tables in PostgreSQL).
There are plenty of tests involving this construct, so I didn't add any
more explicit tests. (But it might be worth adding a test involving
column privileges.)

Thoughts?