Added third server for IP
Added third server for IP

I’ve set up IP checks for both IPv4 and IPv6 on my own Linode VPS.
(ip4.iurl.no -> IPv4 and ip.iurl.no -> IPv6). That makes the total of 3
different IP resolvers for both types. Also added the possibility to
force use of IPv4 for update. See inline comment.

gulbrandsoy/dyndns-linode