Add TRUNCATE command, with psql help and sgml additions.

doc/src/sgml/ref/truncate.sgml

src/backend/catalog/heap.c

src/backend/commands/creatinh.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/backend/tcop/utility.c

src/backend/utils/mmgr/portalmem.c

src/bin/psql/psqlHelp.h

src/include/catalog/heap.h

src/include/commands/creatinh.h

src/include/nodes/nodes.h

src/include/nodes/parsenodes.h

src/include/storage/bufmgr.h

src/include/utils/portal.h

src/tools/pgindent/pgindent