redo/undo support functions and cleanups.

src/backend/access/heap/heapam.c

src/backend/access/nbtree/nbtree.c

src/backend/access/transam/xact.c

src/backend/access/transam/xlog.c

src/backend/access/transam/xlogutils.c

src/backend/storage/buffer/bufmgr.c

src/backend/storage/smgr/md.c

src/include/access/htup.h

src/include/access/xact.h

src/include/access/xlog.h

src/include/catalog/pg_database.h

src/include/storage/buf_internals.h

src/include/storage/bufmgr.h

src/include/storage/bufpage.h

src/include/storage/itemid.h