Restore the vpnkit-userland-proxy

[djs55]

dockerd allows the userland proxy to be overridden with a custom binary. In Docker Desktop we use the LinuxKit pkg/vpnkit-expose-port which is now different from our vpnkit-expose-port. The vpnkit-expose-port in this repo is an example of how to use the vpnkit Go package to manipulate ports; it does not know how to manipulate a mounted 9P filesystem like the pkg/vpnkit-expose-port does.

This PR

• restores the old pkg/vpnkit-expose-port as cmd/vpnkit-userland-proxy (to use the same terminology as Docker)
• sets the -no-local-ip parameter to true to avoid port clashes in the VM

[djs55]

This PR only involves Go code, so it's safe to merge without waiting for appveyor.