log_irssi should emit "--- Day changed" lines

Hm. Let me clarify a bit. The point is that when reading ctrlproxy's irssi-formatted logs, it should be possible to find out with ease what date a particular line belongs to. In real irssi logs, I seach backwards for `/^---/`, which is either a "Log opened DATE" line, or a "Day changed DATE" one.

Regarding ctrlproxy, how does the following sound? The first time a log file is written to, emit a "Log opened DATE" line; store somewhere the timestamp (or, as an optimization, store the timestamp of 0:00 hours of the next day). Then, instead of emiting "Day changed" lines on each day change, it only does so when writing again, if the day has indeed changed since the stored timestamp.

To mimic irssi a bit more closely, another option would be to only emit "Day changed" when writing not after than some sensible amount of time after the previous time we wrote; if not, just emit another "Log opened" line.

(Wow, that was messy, but HTH. Feel free to grab me on IRC if you want.)