split auth from other logging

[Kai Schaetzl]

Is there a way to split the auth logging (logins and failed logins) from 
the other logging that goes to 
info_log_path = /var/log/dovecot/dovecot.log
?
This log gets a lot of other info as well, most notably the lmtp 
notifications about every filed mail (with no level stamping, btw).
This makes it really hard to find authentication errors quickly and 
comfortably.
It would be nice to be able to split at least the lmtp messages away.

Kai