refactor: Share a common helper (vhost builder) for sourcing domains (#2620)

* chore: Split vhost helper method and use filepath vars

- Helpers `accounts.sh` and `aliases.sh` can move their vhost code into this helper.
- They share duplicate code with `bin/open-dkim` which will also leverage this vhost helper going forward.

* chore: Sync vhost generation logic into helper

- Chunky commit, but mostly copy/paste of logic into a common method.
- `bin/open-dkim` additionally wrapped relevant logic in a function call and revised inline docs.

* chore: Include LDAP vhost support

- Revises notes for LDAP vhost support.
- This now ensures LDAP users get vhost rebuilt to match the startup script for when change detection support is enabled.
- `bin/open-dkim` will additionally be able to support the default `DOMAINNAME` var (set via `helpers/dns.sh`) for LDAP users instead of requiring them to provide one explicitly.

* chore(`bin/open-dkim`): Ensure `DOMAINNAME` is properly set

- This will ensure LDAP users insert the same `DOMAINNAME` value as used during container startup.
- The container itself should panic at startup (during `helpers/dns.sh`) if this isn't configured correctly already, thus it should not introduce any breaking change to users of this utility?

* chore: Set the 2nd value as blank `_`

Line is split by a delimiter such as white-space (or via IFS: `|`), the blank `_` var is to indicate we're not interested in that value, but still leverage how `read -r` works, instead of splitting the var ourselves first thing.

* chore: Remove shellcheck disable lines

No longer applicable with the switch to `_`

target/scripts/helpers/aliases.sh

@@ -22,17 +22,6 @@ function _handle_postfix_virtual_config
     fi
 
     cp -f "${DATABASE_VIRTUAL}" /etc/postfix/virtual
-
-    # the `to` is important, don't delete it
-    # shellcheck disable=SC2034
-    while read -r FROM TO
-    do
-      UNAME=$(echo "${FROM}" | cut -d @ -f1)
-      DOMAIN=$(echo "${FROM}" | cut -d @ -f2)
-
-      # if they are equal it means the line looks like: "user1     other@domain.tld"
-      [[ ${UNAME} != "${DOMAIN}" ]] && echo "${DOMAIN}" >>/tmp/vhost.tmp
-    done < <(_get_valid_lines_from_file "${DATABASE_VIRTUAL}")
   else
     _log 'debug' "'${DATABASE_VIRTUAL}' not provided - no mail alias/forward created"
   fi