Fix libpq example programs
When these programs call pg_catalog.set_config, they need to check for
PGRES_TUPLES_OK instead of PGRES_COMMAND_OK. Fix for
5770172cb0c9df9e6ce27c507b449557e5b45124.
Reported-by: Ideriha, Takeshi <ideriha.takeshi@jp.fujitsu.com>