misc: final Rspamd adjustments for v13 (#3599)

* outsource Rspamd ENVs into explicit helper

This will allow us to uniformly source the helper and get the values
from everywhere consistently. This is more than desirable since we will
be using these values not only for the Rspamd setup, but also for DKIM
management and during change-detection.

* integrate Rspamd into changedetection

We outsource one more function to reside in the helper script for Rspamd
so that we can call this function from the Rspamd setup and from the
changedetection functionality too.

* realize deprecation of old commands file for Rspamd

THIS IS A BREAKING CHANGE!

This change realizes the log message: "Using old file location now
(deprecated) - this will prevent startup in v13.0.0" Startup will now
fail.

* added '--force' option to Rspamd DKIM script

* use new helper to get ENVs for Rspamd in DKIM script

* remove the need for linking directories

This was unnecessary, as explained in
https://github.com/docker-mailserver/docker-mailserver/pull/3597#discussion_r1369413599

* Apply suggestions from code review

review by @polarathene

* apply more review feedback from @polarathene

- <https://github.com/docker-mailserver/docker-mailserver/pull/3599#discussion_r1370885519>
- <https://github.com/docker-mailserver/docker-mailserver/pull/3599#discussion_r1370904201>

* update documentation

---------

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

docs/content/config/security/rspamd.md

@@ -107,11 +107,11 @@ DMS brings sane default settings for Rspamd. They are located at `/etc/rspamd/lo
 
 !!! question "What is [`docker-data/dms/config/`][docs-dms-config-volume]?"
 
-If you want to overwrite the default settings and / or provide your own settings, you can place files at `docker-data/dms/config/rspamd/override.d/` (a directory that is linked to `/etc/rspamd/override.d/`, if it exists) to override Rspamd and DMS default settings. This directory will not do a complete file override, but a [forced override of the specific settings in that file][rspamd-docs-override-dir].
+If you want to overwrite the default settings and / or provide your own settings, you can place files at `docker-data/dms/config/rspamd/override.d/`. Files from this directory are copied to `/etc/rspamd/override.d/` during startup. These files [forcibly override][rspamd-docs-override-dir] Rspamd and DMS default settings.
 
 !!! warning "Clashing Overrides"
 
-    Note that when also [using the `rspamd-commands` file](#with-the-help-of-a-custom-file), files in `override.d` may be overwritten in case you adjust them manually and with the help of the file.
+    Note that when also [using the `custom-commands.conf` file](#with-the-help-of-a-custom-file), files in `override.d` may be overwritten in case you adjust them manually and with the help of the file.
 
 [rspamd-docs-override-dir]: https://www.rspamd.com/doc/faq.html#what-are-the-locald-and-overrided-directories
 [docs-dms-config-volume]: ../../faq.md#what-about-the-docker-datadmsconfig-directory