Fix issue with concatenating $dmarc_milter and $dkim_milter in main.cf (#3380)

* Fix issue with concatenating $dmarc_milter and $dkim_milter in main.cf 

Upon each start the  `smtpd_milters` and `non_smtpd_milters` would be extended with the following:
```
smtpd_milters =   $dmarc_milter $dkim_milter
non_smtpd_milters = $dkim_milter
```
In my case they became long enough that mail delivery stopped. I think this was because of the extra headers that are added by these steps. (which in turn would cause the mail to be dropped)

* fix sed to work when the variables are there and when they are not.

---------

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

target/scripts/startup/setup.d/dmarc_dkim_spf.sh

@@ -17,8 +17,8 @@ function _setup_opendkim() {
     postconf 'dkim_milter = inet:localhost:8891'
     # shellcheck disable=SC2016
     sed -i -E                                            \
-      -e 's|^(smtpd_milters =.*)|\1 \$dkim_milter|g'     \
-      -e 's|^(non_smtpd_milters =.*)|\1 \$dkim_milter|g' \
+      -e '/\$dkim_milter/! s|^(smtpd_milters =.*)|\1 \$dkim_milter|g'     \
+      -e '/\$dkim_milter/! s|^(non_smtpd_milters =.*)|\1 \$dkim_milter|g' \
       /etc/postfix/main.cf
 
     # check if any keys are available
@@ -64,7 +64,7 @@ function _setup_opendmarc() {
     postconf 'dmarc_milter = inet:localhost:8893'
     # Make sure to append the OpenDMARC milter _after_ the OpenDKIM milter!
     # shellcheck disable=SC2016
-    sed -i -E 's|^(smtpd_milters =.*)|\1 \$dmarc_milter|g' /etc/postfix/main.cf
+    sed -i -E '/\$dmarc_milter/! s|^(smtpd_milters =.*)|\1 \$dmarc_milter|g' /etc/postfix/main.cf
 
     sed -i \
       -e "s|^AuthservID.*$|AuthservID          ${HOSTNAME}|g" \