-Ystatistics accepts a list of phases for which to print stats

Also cleaned up handling of defaults in MultiChoiceSetting.
Not a beauty, I agree. But as I said, I'd really like to keep this `value` list consistent, it's part of the API of the setting. And it moves the complexity to the right point: when a new setting is parsed, we decide wether or not to add it to the list. Afterwards, querying for enabled options is straightforward.