Explain kicks only to nonguests; only on 2nd join

This is an attempt to reduce the spam that autokick notifications
generate.

Nobody gets an explanation on their first kick.  If you join a second
time, you get an explanation in public chat.  If you join a third time,
you get silently kicked.

Guests get no explanations ever.
Merge pull request #200 from aiannacc/limited-autokick-explanations

Explain kicks only to nonguests; only on 2nd join