Support statement-level ON TRUNCATE triggers. Simon Riggs

doc/src/sgml/plperl.sgml

doc/src/sgml/plpgsql.sgml

doc/src/sgml/plpython.sgml

doc/src/sgml/pltcl.sgml

doc/src/sgml/ref/create_trigger.sgml

doc/src/sgml/ref/truncate.sgml

doc/src/sgml/trigger.sgml

src/backend/commands/tablecmds.c

src/backend/commands/trigger.c

src/backend/executor/execMain.c

src/backend/parser/gram.y

src/backend/utils/adt/ruleutils.c

src/bin/pg_dump/pg_dump.c

src/include/catalog/pg_trigger.h

src/include/commands/trigger.h

src/include/executor/executor.h

src/include/utils/rel.h

src/pl/plperl/plperl.c

src/pl/plpgsql/src/pl_exec.c

src/pl/plpython/plpython.c