fix: Ensure configs are sanitized for parsing (#3819)

* chore: Detect missing final newline in configs read

These lines will be not be processed by `read`, emit a warning to raise awareness.

* fix: Ensure parsed config has final newline appended (when possible)

This functionality was handled in `accounts.sh` via a similar sed command (that the linked references also offer).

`printf` is better for this, no shellcheck comment required either.

We additionally don't attempt to modify files that are read-only.

* fix: Ensure parsed configs have CRLF to LF corrected (where possible)

Likewise, this runtime fix was only covering two config files. It now applies to all callers of this method.

* fix: Sanitize `postfix-master.cf` via helper

This feature should have been using the helper to avoid user error from their config updates accidentally introducing subtle breakage implicitly (due to CRLF or missing final newline).

* tests: Add test cases for new helpers

* tests:  `rm` is redundant when using `BATS_TEST_TMPDIR`

This temporary directory is created and removed implicitly. Even after a test failure.

* chore: Remove old `postfix-virtual.cf` migration logic

This was introduced in 2018, there should be no one needing to rely on this anymore?

* tests: Remove comment on sed failure concern

* chore: Add entry to `CHANGELOG.md`

* Apply suggestions from code review

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

---------

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

CHANGELOG.md

@@ -38,6 +38,10 @@ The most noteworthy change of this release is the update of the container's base
 - **Tests:**
   - Refactored helper methods for sending e-mails with specific `Message-ID` headers and the helpers for retrieving + filtering logs, which together help isolate logs relevant to specific mail when multiple mails have been processed within a single test. ([#3786](https://github.com/docker-mailserver/docker-mailserver/pull/3786))
 
+### Fixes
+
+- DMS config files that are parsed line by line are now more robust to parse by detecting and fixing line-endings ([#3819](https://github.com/docker-mailserver/docker-mailserver/pull/3819))
+
 ## [v13.3.1](https://github.com/docker-mailserver/docker-mailserver/releases/tag/v13.3.1)
 
 ### Fixes