Fix behaviour of menu and depexts in non-interactive environments

[AltGr]

The rationale is now to assume the "no" case in the menus whenever there is no input to read from.

[AltGr]

As mentionned in private, this is more restrictive when running non-interactively but better reflects what I had in mind when adding the menu. I am open to discussion if it has some unwanted effects though, esp. since different workarounds have been proposed in the meantime.

[rjbou]

The new mechanism is good for me! clearer and more local