refactor: Revise check-for-changes.sh (#2615)

- Inline docs for `check-for-changes.sh` have been shuffled around and revised a bit.
- Change processing extracted from the main change detection loop method to their own methods:
   - `_get_changed_files()` - Clarifies what is going on (and how) without having to look it up. To reduce noise in the main logic loop, extracted to a separate method.
   - `_postfix_dovecot_changes()` - The bulk of change processing was moved to this method. I've added conditionals to only run relevant logic.
   - `_ssl_changes()` - Just shifted, no logic changed. `REGEX_NEVER_MATCH` and `ACME_CERT_DIR` vars scope set to `local`.

target/scripts/helpers/aliases.sh

@@ -9,7 +9,6 @@
 function _handle_postfix_virtual_config
 {
   : >/etc/postfix/virtual
-  : >/etc/postfix/regexp
 
   local DATABASE_VIRTUAL=/tmp/docker-mailserver/postfix-virtual.cf
 
@@ -29,6 +28,8 @@ function _handle_postfix_virtual_config
 
 function _handle_postfix_regexp_config
 {
+  : >/etc/postfix/regexp
+
   if [[ -f /tmp/docker-mailserver/postfix-regexp.cf ]]
   then
     _log 'trace' "Adding regexp alias file postfix-regexp.cf"