pgsql: Teach CLUSTER to skip writing WAL if not needed (ie, not using

Log Message:
-----------
Teach CLUSTER to skip writing WAL if not needed (ie, not using archiving)
--- Simon.
Also, code review and cleanup for the previous COPY-no-WAL patches --- Tom.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        perform.sgml (r1.63 -> r1.64)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/perform.sgml.diff?r1=1.63&r2=1.64)
    pgsql/src/backend/access/heap:
        heapam.c (r1.229 -> r1.230)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.229&r2=1.230)
        tuptoaster.c (r1.71 -> r1.72)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/tuptoaster.c.diff?r1=1.71&r2=1.72)
    pgsql/src/backend/catalog:
        index.c (r1.281 -> r1.282)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.281&r2=1.282)
    pgsql/src/backend/commands:
        cluster.c (r1.157 -> r1.158)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c.diff?r1=1.157&r2=1.158)
        copy.c (r1.278 -> r1.279)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c.diff?r1=1.278&r2=1.279)
    pgsql/src/backend/executor:
        execMain.c (r1.291 -> r1.292)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.291&r2=1.292)
    pgsql/src/backend/utils/cache:
        relcache.c (r1.258 -> r1.259)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.258&r2=1.259)
    pgsql/src/include/access:
        heapam.h (r1.120 -> r1.121)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h.diff?r1=1.120&r2=1.121)
        tuptoaster.h (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/tuptoaster.h.diff?r1=1.32&r2=1.33)
    pgsql/src/include/utils:
        rel.h (r1.99 -> r1.100)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/rel.h.diff?r1=1.99&r2=1.100)
        relcache.h (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/relcache.h.diff?r1=1.58&r2=1.59)