proxy: add interactive mode with `-i`
proxy: add interactive mode with `-i`

This allows the proxy to be run easily from a terminal or other script
without requiring fd 3 to be open and writable.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit