[Dovecot] LMTP Logging

Just had a minor suggestion, with no clue how hard/easy it would be to implement:

The %f flag in deliver_log_format seems to pick up the From: header, instead of the "MAIL FROM:<...>" arg. It'd be handy to have a %F that shows the "MAIL FROM" arg instead. I'm looking at tracking emails through logs from Exim to Dovecot easily. I know Message-ID can be used for correlation but it adds some complexity to searching, i.e. I can't just grep for the sender (as logged by Exim), unless I assume "MAIL FROM" always == From: