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)