Refactor broken CREATE TABLE IF NOT EXISTS support.

Per bug #5988, reported by Marko Tiikkaja, and further analyzed by Tom
Lane, the previous coding was broken in several respects: even if the
target table already existed, a subsequent CREATE TABLE IF NOT EXISTS
might try to add additional constraints or sequences-for-serial
specified in the new CREATE TABLE statement.

In passing, this also fixes a minor information leak: it's no longer
possible to figure out whether a schema to which you don't have CREATE
access contains a sequence named like "x_y_seq" by attempting to create a
table in that schema called "x" with a serial column called "y".

Some more refactoring of this code in the future might be warranted,
but that will need to wait for a later major release.

src/backend/commands/cluster.c

@@ -646,8 +646,7 @@ make_new_heap(Oid OIDOldHeap, Oid NewTableSpace)
 										  ONCOMMIT_NOOP,
 										  reloptions,
 										  false,
-										  true,
-										  false);
+										  true);
 	Assert(OIDNewHeap != InvalidOid);
 
 	ReleaseSysCache(tuple);