change TCP protoinfo via netlink

Taken from [1]:

I think that users must be able to set the TCP state of a conntrack, otherwise if they create a conntrack via libnfnetlink_conntrack, the state will be set to NONE. If users aren't able to set the state, I'd consider that the library is kind of incomplete. I don't like the idea of kidding with stuff that could be compromising either, but I don't want to lose any features. So, I think that the solution is *remark* in the documentation of libnfnetlink_conntrack and the conntrack manpage that changing the protocol private information like the state could result in problems, I'll to do such thing.

[1]https://lists.netfilter.org/pipermail/netfilter-devel/2005-August/020784.html

@Harald: I've reworked the patches as you suggested.

--
Pablo