Add a simple control-file protocol for port forwards
Add a simple control-file protocol for port forwards

To create a forward:

  echo local_port:remote_ip:remote_port > ctl
  result = $(read ctl)

the result will either start with "OK local_port:remote_ip:remote_port"
or "ERROR message"

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

djs55/vpnkit,djs55/vpnkit,samoht/vpnkit,djs55/vpnkit