Improve logging of checkpoints. Patch by Greg Smith, worked over

doc/src/sgml/config.sgml

src/backend/access/transam/xlog.c

src/backend/postmaster/bgwriter.c

src/backend/storage/buffer/bufmgr.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/include/access/xlog.h