Add inadyn username and password if they don't exist

src/freedombone-utils-dns

     else
         # change username/password for an existing provider
         if [ "$DDNS_USERNAME" ]; then
-            sed -i "s|  username .*|  username $DDNS_USERNAME|g" /etc/inadyn.conf
+            if grep -q "  username " /etc/inadyn.conf; then
+                sed -i "s|  username .*|  username $DDNS_USERNAME|g" /etc/inadyn.conf
+            else
+                echo "  username $DDNS_USERNAME" >> /etc/inadyn.conf
+            fi
         fi
         if [ "$DDNS_PASSWORD" ]; then
-            sed -i "s|  password .*|  password $DDNS_PASSWORD|g" /etc/inadyn.conf
+            if grep -q "  password " /etc/inadyn.conf; then
+                sed -i "s|  password .*|  password $DDNS_PASSWORD|g" /etc/inadyn.conf
+            else
+                echo "  password $DDNS_PASSWORD" >> /etc/inadyn.conf
+            fi
         fi
     fi