Customize log format

[Mathieu-Lala]

With tracing it is possible to change the format of the generated output. This format should be customization in the configuration

[paguardiolle]

Before production release:

1. [x] Review error/warn and info levels. Not enough information (warn) and/or too much.
2. [ ] Follow historical guidelines In "TXT" mode the default. Output must be more concise (except for trace and debug).
3. [x] Follow Unix FHS and best practices : vSMTP main log files must be in /var/log/vsmtp w/o a timestamp (ie vsmtp.log and app.log) - ok for the archives in subdirectoris
4. [x] Syslog : there must be an option (goal : logs in /var/log/mail.log and mail.err)