Function share_one 

fn share_one(
    card: &KnownCard,
    own: &Path,
    shared: &Path,
    adds: bool,
) -> Result<(), String>

One security key’s shared file written from this device: what cards.toml says of it, and its certificate merged with the one shared already, so that a renewal made on another computer is never undone. Written only when it changed; with adds, only when this device’s certificate adds to the shared one (the record itself left as written).