DECLARE CURSOR to become utility statement

I'm about to change the parsetree representation of DECLARE CURSOR
to look more like a standard utility statement: a DeclareCursorStmt
node with a SELECT querytree dangling from it.  This will make it
possible to remove the cursor-specific elements of Query nodes
(isPortal, isBinary), rather than adding more as we'd ultimately need
to do if we want to have SCROLL, INSENSITIVE, etc do anything.

Any objections?
        regards, tom lane