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/include/access/rmgr.h

@@ -3,7 +3,7 @@
  *
  * Resource managers definition
  *
- * $PostgreSQL: pgsql/src/include/access/rmgr.h,v 1.20 2009/12/19 01:32:42 sriggs Exp $
+ * $PostgreSQL: pgsql/src/include/access/rmgr.h,v 1.21 2010/02/07 20:48:11 tgl Exp $
  */
 #ifndef RMGR_H
 #define RMGR_H
@@ -23,6 +23,7 @@ typedef uint8 RmgrId;
 #define RM_DBASE_ID				4
 #define RM_TBLSPC_ID			5
 #define RM_MULTIXACT_ID			6
+#define RM_RELMAP_ID			7
 #define RM_STANDBY_ID			8
 #define RM_HEAP2_ID				9
 #define RM_HEAP_ID				10