conntrackd: userspace SSDP helper

Here is a patch which adds a userspace conntrack helper for the SSDP
protocol. This is based on the code found at:

http://marc.info/?t=132945775100001&r=1&w=2

I'm not sure how to get my laptop to play at IPv6, so I've not tested
this part, but I've tested the IPv4 section and it works.

Signed-off-by: Ash Hughes <ashley.hughes@blueyonder.co.uk>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>