netfilter: nft_exthdr: add boolean DCCP option matching

The xt_dccp iptables module supports the matching of DCCP packets based
on the presence or absence of DCCP options.  Extend nft_exthdr to add
this functionality to nftables.

Link: https://bugzilla.netfilter.org/show_bug.cgi?id=930


Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>