implement network debug logging

[corecode]

[corecode]

@bapt would appreciate feedback, especially regarding security concerns.

I require the debug log file to already exist, which allows the admin to set permissions accordingly.

[corecode]

I implemented this to debug the server conversation without leaking data into syslog. I don't think we need it much, but I needed it to see whether there was something weird going on during successful delivery of a mail. I prefer not to have this functionality in the code, but I suspect we might need it occasionally...