ddns-scripts: correct freedns.afraid.org update url and new provider
- roll back to "old" update_url of freedns.afraid.org fix for #2445 and openwrt/luci#661
- add provider nubem.com (IPv4 only)
- add provider nettica.com (IPv4 only)
- add provider zerigo.com (IPv4 and IPv6)
- add provider regfish.de (IPv4 and IPv6)
Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>