fix: check-for-changes.sh should not fall out of sync with shared logic (#2260)

Removes duplicate logic from `check-for-changes.sh` that is used/maintained elsewhere to avoid risk of problems, as this code is already starting to diverge / rot.

---

Previously the change detection support has had code added for rebuilding config upon change detection which is the same as code run during startup scripts. Unfortunately over time this has fallen out of sync. Mostly the startup scripts would get maintenance and the contributor and reviewers may not have been aware of the duplicate code handled by `check-for-changes.sh`.

That code was starting to diverge in addition to some changes in structure (_eg: relay host logic seems interleaved here vs separated out in startup scripts_). I wanted to address this before it risks becoming a much bigger headache.

Rather than bloat `helper-functions.sh` further, I've added a `helpers/` folder extracting relevant common logic between startup scripts and `changedetector`. If you want to follow that process I've kept scoped commits to make those diffs easier. Some minor changes/improvements were added but nothing significant.

---

- chore: Extract relay host logic to new `relay.sh` helper
- chore: Extract `/etc/postfix/sasl_passwd` logic to new `sasl.sh` helper
- chore: Extract `postfix-accounts.cf` logic to new `accounts.sh` helper
- chore: Extract `/etc/aliases` logic to new `aliases.sh` helper
- chore: Extract `/etc/postfix/vhost` logic to new `postfix.sh` helper

- chore: Add inline docs for Postfix configs
> These are possibly more verbose than needed and can be reduced at a later stage.
> They are helpful during this refactor process while investigating that everything is handled correctly.

`accounts.sh`: 
- Add note regarding potential bug for bare domain setups with `/etc/postfix/vhost` and `mydestination` sharing same domain value.

`relay.sh`: 
- Remove the tabs for a single space delimiter, revised associated comment.
- Add PR reference for original `_populate_relayhost_map` implementation which has some useful details.


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

target/scripts/helper-functions.sh

@@ -1,5 +1,10 @@
 #! /bin/bash
 
+# These helpers are used by `setup-stack.sh` and `check-for-changes.sh`,
+# not by anything within `helper-functions.sh` itself:
+# shellcheck source=target/scripts/helpers/index.sh
+. /usr/local/bin/helpers/index.sh
+
 DMS_DEBUG="${DMS_DEBUG:=0}"
 SCRIPT_NAME="$(basename "$0")" # This becomes the sourcing script name (Example: check-for-changes.sh)
 LOCK_ID="$(uuid)" # Used inside of lock files to identify them and prevent removal by other instances of docker-mailserver
@@ -213,44 +218,6 @@ function _notify
 }
 export -f _notify
 
-# ? --------------------------------------------- Relay Host Map
-
-# setup /etc/postfix/relayhost_map
-# --
-# @domain1.com        [smtp.mailgun.org]:587
-# @domain2.com        [smtp.mailgun.org]:587
-# @domain3.com        [smtp.mailgun.org]:587
-function _populate_relayhost_map
-{
-  : >/etc/postfix/relayhost_map
-  chown root:root /etc/postfix/relayhost_map
-  chmod 0600 /etc/postfix/relayhost_map
-
-  if [[ -f /tmp/docker-mailserver/postfix-relaymap.cf ]]
-  then
-    _notify 'inf' "Adding relay mappings from postfix-relaymap.cf"
-    # keep lines which are not a comment *and* have a destination.
-    sed -n '/^\s*[^#[:space:]]\S*\s\+\S/p' /tmp/docker-mailserver/postfix-relaymap.cf >> /etc/postfix/relayhost_map
-  fi
-
-  {
-    # note: won't detect domains when lhs has spaces (but who does that?!)
-    sed -n '/^\s*[^#[:space:]]/ s/^[^@|]*@\([^|]\+\)|.*$/\1/p' /tmp/docker-mailserver/postfix-accounts.cf
-
-    [ -f /tmp/docker-mailserver/postfix-virtual.cf ] && sed -n '/^\s*[^#[:space:]]/ s/^\s*[^@[:space:]]*@\(\S\+\)\s.*/\1/p' /tmp/docker-mailserver/postfix-virtual.cf
-  } | while read -r DOMAIN
-  do
-    # DOMAIN not already present *and* not ignored
-    if ! grep -q -e "^@${DOMAIN}\b" /etc/postfix/relayhost_map && ! grep -qs -e "^\s*@${DOMAIN}\s*$" /tmp/docker-mailserver/postfix-relaymap.cf
-    then
-      _notify 'inf' "Adding relay mapping for ${DOMAIN}"
-      # shellcheck disable=SC2153
-      echo "@${DOMAIN}    [${RELAY_HOST}]:${RELAY_PORT}" >> /etc/postfix/relayhost_map
-    fi
-  done
-}
-export -f _populate_relayhost_map
-
 # ? --------------------------------------------- File Checksums
 
 # file storing the checksums of the monitored files.
@@ -349,6 +316,12 @@ function _obtain_hostname_and_domainname
   DOMAINNAME="${DOMAINNAME:-${HOSTNAME}}"
 }
 
+# Remove string input with empty line, only whitespace or `#` as the first non-whitespace character.
+function _strip_comments
+{
+  grep -q -E "^\s*$|^\s*#" <<< "${1}"
+}
+
 # Call this method when you want to panic (emit a 'FATAL' log level error, and exit uncleanly).
 # `dms_panic` methods should be preferred if your failure type is supported.
 function _shutdown