Please allow postfix to log to a file

[yurividal]

Hi.
Please add an environment variable to allow setting postfix to log to a file.
Currently, main.cf defaults to log to /dev/stdout
By allowing postfix to log to a file instead, the user may be able to mount the log file to the host machine, in order to use tools like fail2ban and crowdsec (which need clean postfix logs in order to parse them properly).