extension: libip6t_DNAT: allow port DNAT without address

correct parsing of IPv6 port NAT without address NAT,
assume one colon as port information.

Allows:
* address only:
 -j DNAT --to affe::1
 -j DNAT --to [affe::1]

* port only
 -j DNAT --to :80
 -j DNAT --to :80-110
 -j DNAT --to []:80
 -j DNAT --to []:80-110

* address and port
 -j DNAT --to [affe::1]:80
 -j DNAT --to [affe::1]:80-110

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
extensions: libip6t_DNAT: set IPv6 DNAT --to-destination

as in IPv4 and fixes DNAT_save

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>