1393: Ignore newlines and comment-lines in postfix overrides r=mergify[bot] a=Nebukadneza
## What type of PR?
enhancement
## What does this PR do?
To make postfix override files understandable and readable, users may
want to insert empty newlines and #-commented lines in their postfix
override files too. This will now ignore such bogus-lines and not send
them to `postconf`, which produced ugly errors in the past.
### Related issue(s)
closes#1098
## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
Co-authored-by: Dario Ernst <github@kanojo.de>
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