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.

doc/src/sgml/pgbuffercache.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/pgbuffercache.sgml,v 2.5 2009/05/18 11:08:24 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/pgbuffercache.sgml,v 2.6 2010/02/07 20:48:09 tgl Exp $ -->
 
 <sect1 id="pgbuffercache">
  <title>pg_buffercache</title>
@@ -56,7 +56,7 @@
       <entry><structfield>relfilenode</structfield></entry>
       <entry><type>oid</type></entry>
       <entry><literal>pg_class.relfilenode</literal></entry>
-      <entry>Relfilenode of the relation</entry>
+      <entry>Filenode number of the relation</entry>
      </row>
 
      <row>
@@ -137,7 +137,7 @@
   <programlisting>
   regression=# SELECT c.relname, count(*) AS buffers
                FROM pg_buffercache b INNER JOIN pg_class c
-               ON b.relfilenode = c.relfilenode AND
+               ON b.relfilenode = pg_relation_filenode(c.oid) AND
                   b.reldatabase IN (0, (SELECT oid FROM pg_database
                                         WHERE datname = current_database()))
                GROUP BY c.relname