IPV6 addresses containing a double colon `::` two times are treated as valid.
This enables users to configure wrong IPv6 addresses via the VyOS CLI which ises this validator.
```
root@LR1:~# ipaddrcheck --is-ipv6 2001:db8::1
root@LR1:~# echo $?
0
root@LR1:~# ipaddrcheck --is-ipv6 2001:db8::g
root@LR1:~# echo $?
1
root@LR1:~# ipaddrcheck --is-ipv6 2001:db8::1/64
root@LR1:~# echo $?
0
root@LR1:~# ipaddrcheck --is-ipv6 2001:db8::/64
root@LR1:~# echo $?
0
root@LR1:~# ipaddrcheck --is-ipv6 2001::db8::1/64
root@LR1:~# echo $?
0
```