pgsql: Improve logging of checkpoints.

Log Message:
-----------
Improve logging of checkpoints. Patch by Greg Smith, worked over
by Heikki and a little bit by me.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.129 -> r1.130)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.129&r2=1.130)
pgsql/src/backend/access/transam:
xlog.c (r1.273 -> r1.274)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.273&r2=1.274)
pgsql/src/backend/postmaster:
bgwriter.c (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.39&r2=1.40)
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.222 -> r1.223)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.222&r2=1.223)
pgsql/src/backend/utils/misc:
guc.c (r1.403 -> r1.404)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.403&r2=1.404)
postgresql.conf.sample (r1.217 -> r1.218)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.217&r2=1.218)
pgsql/src/include/access:
xlog.h (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h.diff?r1=1.79&r2=1.80)