BAN: Don't schedule check_klines for 0 seconds in the future.

When receiving bans from a bursting server, if kline_delay is set to 0
(the default), rb_event_addonce will be called to schedule an event for
0 seconds in the future. While this works fine for the fallback
rb_event_run function, the epoll implementation ends up scheduling a
timerfd for the event in the past, which is then never executed.

While fixing this, I also made rb_event_add and rb_event_addonce reject
attempts to add events scheduled for 0 seconds in the future; they're
instead rewritten to run 1 second in the future.