[Config] Add MAILDIR_SUB, "Maildir" for new setups by default

[Update] Add MAILDIR_SUB= for updated mailcows
[Dovecot] Read MAILDIR_SUB for mail_home

update.sh

@@ -243,6 +243,12 @@ for option in ${CONFIG_ARRAY[@]}; do
       echo '# Disable Solr or if you do not want to store a readable index of your mails in solr-vol-1.' >> mailcow.conf
       echo "SKIP_SOLR=y" >> mailcow.conf
   fi
+  elif [[ ${option} == "MAILDIR_SUB" ]]; then
+    if ! grep -q ${option} mailcow.conf; then
+      echo "Adding new option \"${option}\" to mailcow.conf"
+      echo '# MAILDIR_SUB defines a path in a users virtual home to keep the maildir in. Leave empty for updated setups.' >> mailcow.conf
+      echo "MAILDIR_SUB=" >> mailcow.conf
+  fi
   elif ! grep -q ${option} mailcow.conf; then
     echo "Adding new option \"${option}\" to mailcow.conf"
     echo "${option}=n" >> mailcow.conf