Create a "relation mapping" infrastructure to support changing the relfilenodes

of shared or nailed system catalogs.  This has two key benefits:

* The new CLUSTER-based VACUUM FULL can be applied safely to all catalogs.

* We no longer have to use an unsafe reindex-in-place approach for reindexing
  shared catalogs.

CLUSTER on nailed catalogs now works too, although I left it disabled on
shared catalogs because the resulting pg_index.indisclustered update would
only be visible in one database.

Since reindexing shared system catalogs is now fully transactional and
crash-safe, the former special cases in REINDEX behavior have been removed;
shared catalogs are treated the same as non-shared.

This commit does not do anything about the recently-discussed problem of
deadlocks between VACUUM FULL/CLUSTER on a system catalog and other
concurrent queries; will address that in a separate patch.  As a stopgap,
parallel_schedule has been tweaked to run vacuum.sql by itself, to avoid
such failures during the regression tests.

src/backend/executor/execMain.c

@@ -26,7 +26,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.344 2010/02/03 10:01:30 heikki Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.345 2010/02/07 20:48:10 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2168,6 +2168,7 @@ OpenIntoRel(QueryDesc *queryDesc)
 											  NIL,
 											  RELKIND_RELATION,
 											  false,
+											  false,
 											  true,
 											  0,
 											  into->onCommit,