scripts: Rspamd stabilization pt. 1 (#3261)

* added checks whether OpenDKIM/OpenDMARC/policyd-spf are enabled
* added functions to check if VAR is 0/0 or an int

and also added tests.

I also adjusted the test file to not run in a container, because there
is no need. This also decreases test time, which, in turn, increases
maintainers' happiness.

* added more checks to Rspamd setup

I added the helpers from the previous commit to the Rspamd setup to make
the whole setup more robust, and indicate to the user that an ENV
variable's value is incorrect.

While we did not issues for this in the past, I believe it to be
worthwhile for the future.

* added canonical directory for users to place files in

This dir is canonical with DMS's optional configuration dirs, as it
lives in well-known volume mounts. Hence, users will not need to adjust
`/etc/rspamd/override.d` manually anymore, or mount a volume to this
place.

The docs explain this now, but the DKIM page needs a slight update on
this too I guess. I will follow-up here.

* misc minor improvements
* use variables for common directories

docs/content/config/advanced/optional-config.md

@@ -12,6 +12,7 @@ This is a list of all configuration files and directories which are optional or
 - **sieve-pipe:** directory for sieve pipe scripts. (Docs: [Sieve][docs-sieve])
 - **opendkim:** DKIM directory. Auto-configurable via [`setup.sh config dkim`][docs-setupsh]. (Docs: [DKIM][docs-dkim])
 - **ssl:** SSL Certificate directory if `SSL_TYPE` is set to `self-signed` or `custom`. (Docs: [SSL][docs-ssl])
+- **Rspamd:** Override directory for custom settings when using Rspamd (Docs: [Rspamd][docs-rspamd-override-d])
 
 ## Files
 
@@ -54,5 +55,6 @@ This is a list of all configuration files and directories which are optional or
 [docs-sieve]: ./mail-sieve.md
 [docs-setupsh]: ../../config/setup.sh.md
 [docs-ssl]: ../../config/security/ssl.md
+[docs-rspamd-override-d]: ../security/rspamd.md#manually
 [docs-rspamd-commands]: ../security/rspamd.md#with-the-help-of-a-custom-file
 [github-commit-setup-stack.sh-L411]: https://github.com/docker-mailserver/docker-mailserver/blob/941e7acdaebe271eaf3d296b36d4d81df4c54b90/target/scripts/startup/setup-stack.sh#L411

docs/content/config/security/rspamd.md

@@ -101,17 +101,20 @@ The [RBL module](https://rspamd.com/doc/modules/rbl.html) is enabled by default.
 
 ## Providing Custom Settings & Overriding Settings
 
+DMS brings sane default settings for Rspamd. They are located at `/etc/rspamd/local.d/` inside the container (or `target/rspamd/local.d/` in the repository).
+
 ### Manually
 
-DMS brings sane default settings for Rspamd. They are located at `/etc/rspamd/local.d/` inside the container (or `target/rspamd/local.d/` in the repository). If you want to change these settings and / or provide your own settings, you can
+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.
 
-1. place files at `/etc/rspamd/override.d/` which will override Rspamd settings and DMS settings
-2. (re-)place files at `/etc/rspamd/local.d/` to override DMS settings and merge them with Rspamd settings
+!!! note "What is [`docker-data/dms/config/`][docs-dms-config-volume]?"
 
 !!! 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.
 
+[docs-dms-config-volume]: ../../faq.md#what-about-the-docker-datadmsmail-state-folder
+
 ### With the Help of a Custom File
 
 DMS provides the ability to do simple adjustments to Rspamd modules with the help of a single file. Just place a file called `rspamd-modules.conf` into the [local config directory `docker-data/dms/config/`][docs-volumes-config]. If this file is present, DMS will evaluate it. The structure is _very_ simple. Each line in the file looks like this: