vpnkit-forwarder: new host implementation, new protocol

[djs55]

Previously the 9P host side was inside the main vpnkit binary and used a synthetic 9P filesystem. This was sub-optimal because

• we had to implement all the forwarding protocol twice, once in OCaml and once in Go
• we had to duplicate many test cases
• 9P is not used anywhere else for control interfaces so it's a bit "surprising"
• we needed 2 9P clients: one via a Linux mount and one via a Unix domain socket

This patch implements the host side of the port-forwarding system in Go and replaces the 9P protocol with a simple HTTP based one.

The control plane over HTTP can be exposed over both AF_VSOCK and as a Unix domain socket/ named pipe for maximum convenience.

The data plane can be connected either host -> VM or VM -> host, depending on your preferences (and the stability of your AF_VSOCK implementation)