DDNS: Fix API call for the "Dynu" DDNS service

Note: This PR solves Bugzilla issue [#10556](https://bugzilla.ipfire.org/show_bug.cgi?id=10556)

This affects the DDNS service "dynu.ca dyn.ee dynserv.(ca|org|net|com)".
DNS updates using this service were made using the URL http://dynserv.ca/ . However,
the domain dynserv.ca does not exist anymore. The Dynu service is now only reachable
via the dynu.com domain. This commit changes the API call according to Dynu's
specification on http://www.dynu.com/Default.aspx?page=dnsapi .