cmd/podman: don't warn on `podman run -ti -d` from not a tty

`podman run -ti` warns when stdin is not a tty, but if the container is
run in detached state that warning does not make much sense: we just
need the environment where podman attach will be run to be a tty.

Running with `-ti` even in detached state can make sense to avoid
applications buffering their output (for realtime logs) or allowing
later interaction and should not warn users.

Also remove the comment saying that warning will become fatal, as there
seems to be agreement that such a breaking change will not be made

Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>