Multipath TCP Support

[xvzf]

Do you have any obligations against supporting Multipath TCP for the transport? I'm looking forward to use this as a "proxy" for wireguard over mTCP (I know, kinda hacky setup)

If it's okay for you, I'd add mTCP support that came with the newer kernel revisions

[rfc1036]

I would love to see a patch to implement it. Please add appropriate #ifdefs for platforms which do not implement it.