netfilter: exthdr: tcp option set support

This allows setting 2 and 4 byte quantities in the tcp option space.
Main purpose is to allow native replacement for xt_TCPMSS to
work around pmtu blackholes.

Writes to kind and len are now allowed at the moment, it does not seem
useful to do this as it causes corruption of the tcp option space.

We can always lift this restriction later if a use-case appears.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>