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/utils/rel.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/rel.h,v 1.121 2010/02/04 00:09:14 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/utils/rel.h,v 1.122 2010/02/07 20:48:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -339,6 +339,16 @@ typedef struct StdRdOptions
 #define RelationGetNamespace(relation) \
 	((relation)->rd_rel->relnamespace)
 
+/*
+ * RelationIsMapped
+ *		True if the relation uses the relfilenode map.
+ *
+ * NB: this is only meaningful for relkinds that have storage, else it
+ * will misleadingly say "true".
+ */
+#define RelationIsMapped(relation) \
+	((relation)->rd_rel->relfilenode == InvalidOid)
+
 /*
  * RelationOpenSmgr
  *		Open the relation at the smgr level, if not already done.