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/bootstrap/bootparse.y

@@ -247,8 +247,7 @@ Boot_CreateStmt:
 													  ONCOMMIT_NOOP,
 													  (Datum) 0,
 													  false,
-													  true,
-													  false);
+													  true);
 						elog(DEBUG4, "relation created with oid %u", id);
 					}
 					do_end();