2529: Improve fetchmail r=mergify[bot] a=nextgens
## What type of PR?
enhancement
## What does this PR do?
Improve fetchmail:
- allow delivery via LMTP (faster, bypassing the filters)
- allow several folders to be retrieved
- run fetchmail as non-root
- tweak the compose file to ensure we have all the dependencies
### Related issue(s)
- closes#1231
- closes#2246
- closes#711
## Prerequisites
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.
- [ ] 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/workflow.html#changelog) entry file.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>
Co-authored-by: Florent Daigniere <nextgens@users.noreply.github.com>
