Stricter Limit parsing

[alexsnaps]

We ignore max_value as per #87 … but since we use the same (de)serialization mechanism for data coming in (limits file) and data going out (to the counter storage), this comes with a sad side effect:

You can leave the max_value out of your limits file and it will default to 0 which is probably not what one would ever want. I could still validate that when loading limits from the file, they have a non-negative value as a separate validation… but with this change, at least if someone misspells max_value (e.g. maxvalue, which happened to me), the parsing will fail with a good explanation.

    #[serde(skip_serializing, default)]
    max_value: i64,