* only add Amavis configuration to Postfix when enabled
Since I am running Rspamd nowadays, I noticed there still are ports open
that belong to Amavis. This is because the Amavis configuration is a
fixed part of Postfix's `master.cf`. I changed that. Now, the Amavis
section is added when Amavis really is enabled.
I took the chance and added proper indentation to `master.cf`; hence the
diff is a bit fuzzy. **But**, only the Amavis part was adjusted, the
rest is just styling.
