mptcp: add return value to init_req

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: use a counter to generate nonces

Use the already existing mptcp_key_seed (renamed mptcp_seed for consistency)
to generate nonces. This does not reduce the security of MPTCP as the nonce
generated for the same 5-tuple will still be different.

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: rework mptcp_join_request

This patch removes the code duplication between the tcp and mptcp stack
by reusing the newly introduced tcp_conn_request.

As for Octavian's rework of mptcp_conn_request, the mptcp options are
parsed twice which adds some overhead.

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>