[Dovecot] lmtp proxy logging

[Jack Bates]

The logging on lmtp and lmtp proxy is pretty limited from what I can 
see. It seems to handle errors, Connect, Disconnect, and in the case of 
lmtp delivery, it logs where an email is saved to. The lmtp may be 
enough, "connect, saved user, saved user..., disconnect", but I was 
curious if it is worth while to add more info logging for the proxy, 
primarily which recipients are sent to which proxy. I was thinking of 
local patching it, but I'll generate up something more inline with 
official code if it is desired.

My thought is to show 1 entry for each recipient, and the destination 
server chosen. If I recall correctly, the proxy code doesn't actually 
listen in on the conversation, so logging results would probably 
complicate the code.

Jack