Make json_object_add_value_string() duplicate its 'value' argument

Make json_object_add_value_string() duplicate its 'value' argument

Having some json_object_add_value_string() callers duplicate the 'value'
argument others not is dubious because it can lead to dangling pointers,
e.g. when passing strings to json_object_add_value_string() that live less
long than the created JSON object. See e.g. the 'time_buf' stack array that
is passed by fio_client_json_init() to json_object_add_value_string().

Signed-off-by: Bart Van Assche <bvanassche@acm.org>