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)