Adds /pkg/vpnkit-forwarder which downloads and builds vpnkit-forwarder
and vpnkit-expose-port from moby/vpnkit. Also includes an example for
forwarding `sshd` and updates the documentation for `hyperkit` to
include a `vpnkit` section.
`vpnkit` still requires a 9P mount for coordination, so the `socat`
forwarding is simpler to set up at the moment.
Signed-off-by: Magnus Skjegstad <magnus@skjegstad.com>