Explain kicks only to nonguests; only on 2nd join

Addresses kick explanation spam: #188

I'm not personally convinced that explaining kicks in public chat is a good idea at all... it's so annoying that I myself turned the feature off! But it's either that, drop the Isotropish level autokick, or people just get kicked without figuring out why.

Explaining only to non-guests and only on their 2nd join attempt seemed like a good compromise to me and to yed. Note that they shouldn't get any explanation on their initial or 3rd+ attempt.