Support multi-targetted ModifyTable in plan output in text-style

Support multi-targetted ModifyTable in plan output in text-style

PostgreSQL 9.5 newly supports explicit target table listing for
ModifyTable and pg_store_plans doesn't show such plans sainly in text
style. This doesn't affect JSON, XML, YAML styles of output.

Note: JSON representation lacks information about whether every target
table is a Foreign relation or not so the result of
pg_store_plans_textplan tells as if all target tables are local
relations even if there's remote ones.