Multipath TCP (RFC 6824) support

This commit adds the support of Multipath TCP (MPTCP). MPTCP is a new
extension to TCP standardized at the IETF. MPTCP allows to use several IP
addresses at the same time by distributing data across several subflows (TCP
connections) while still presenting the standard TCP socket API to the
application. Its benefits are better resource utilization, better throughput
and smoother reaction to failures.