extensions: S/DNPT: add missing save function

Jean-Michel DILLY reports that `ip6tables -S` exits with

	Target `DNPT' is missing save function

when a DNPT rule is invoked. Fix this omission.

References: http://marc.info/?l=netfilter&m=135904831220440&w=2
Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Merge branch 'stable' into 'master'
doc: document nat table for IPv6

Based on the IPv4 description.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>