netfilter: xtables: support arpt_mark and ipv6 optstrip for iptables-nft only builds

Its now possible to build a kernel that has no support for the classic
xtables get/setsockopt interfaces and builtin tables.

In this case, we have CONFIG_IP6_NF_MANGLE=n and
CONFIG_IP_NF_ARPTABLES=n.

For optstript, the ipv6 code is so small that we can enable it if
netfilter ipv6 support exists. For mark, check if either classic
arptables or NFT_ARP_COMPAT is set.

Fixes: a9525c7f ("netfilter: xtables: allow xtables-nft only builds")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>