Add CASCADE option to TRUNCATE. Joachim Wieland

doc/src/sgml/ref/truncate.sgml

src/backend/catalog/heap.c

src/backend/commands/tablecmds.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/tcop/utility.c

src/include/catalog/heap.h

src/include/commands/tablecmds.h

src/include/nodes/parsenodes.h

src/test/regress/expected/truncate.out

src/test/regress/sql/truncate.sql