explicitely backup main.cf configuration even though we don't replace

it totally

See: #1029

install/alternc.install

@@ -156,6 +156,8 @@ for file in $CONFIG_FILES; do
     fi
 done
 
+# also backup main.cf since we're doing major changes to it
+BACKUPS="$BACKUPS etc/postfix/main.cf"
 tar -zcf "$BACKUP_FILE" -C / $BACKUPS 2>/dev/null || true
 
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