pgsql: Add some instrumentation to the bgwriter, through the stats

Log Message:
-----------
Add some instrumentation to the bgwriter, through the stats collector.
New view pg_stat_bgwriter, and the functions required to build it.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        monitoring.sgml (r1.47 -> r1.48)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml.diff?r1=1.47&r2=1.48)
    pgsql/src/backend/catalog:
        system_views.sql (r1.36 -> r1.37)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/system_views.sql.diff?r1=1.36&r2=1.37)
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.36 -> r1.37)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.36&r2=1.37)
        pgstat.c (r1.151 -> r1.152)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.151&r2=1.152)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.215 -> r1.216)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.215&r2=1.216)
    pgsql/src/backend/utils/adt:
        pgstatfuncs.c (r1.40 -> r1.41)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c.diff?r1=1.40&r2=1.41)
    pgsql/src/include:
        pgstat.h (r1.56 -> r1.57)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.56&r2=1.57)
    pgsql/src/include/catalog:
        catversion.h (r1.396 -> r1.397)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.396&r2=1.397)
        pg_proc.h (r1.451 -> r1.452)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.451&r2=1.452)
    pgsql/src/test/regress/expected:
        rules.out (r1.126 -> r1.127)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rules.out.diff?r1=1.126&r2=1.127)