Modify processing of DECLARE CURSOR and EXPLAIN so that they can resolve the

types of unspecified parameters when submitted via extended query protocol.
This worked in 8.2 but I had broken it during plancache changes.  DECLARE
CURSOR is now treated almost exactly like a plain SELECT through parse
analysis, rewrite, and planning; only just before sending to the executor
do we divert it away to ProcessUtility.  This requires a special-case check
in a number of places, but practically all of them were already special-casing
SELECT INTO, so it's not too ugly.  (Maybe it would be a good idea to merge
the two by treating IntoClause as a form of utility statement?  Not going to
worry about that now, though.)  That approach doesn't work for EXPLAIN,
however, so for that I punted and used a klugy solution of running parse
analysis an extra time if under extended query protocol.

src/include/commands/portalcmds.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/commands/portalcmds.h,v 1.22 2007/03/13 00:33:43 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/commands/portalcmds.h,v 1.23 2007/04/27 22:05:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -15,10 +15,11 @@
 #define PORTALCMDS_H
 
 #include "nodes/parsenodes.h"
+#include "nodes/plannodes.h"
 #include "utils/portal.h"
 
 
-extern void PerformCursorOpen(DeclareCursorStmt *stmt, ParamListInfo params,
+extern void PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 							  const char *queryString, bool isTopLevel);
 
 extern void PerformPortalFetch(FetchStmt *stmt, DestReceiver *dest,