Interpret port-max-idle-time <= 0 as never timeout port
Interpret port-max-idle-time <= 0 as never timeout port

This allows us to completely disable the mechanism by setting the
port-max-idle-time to 0 (or -ve)

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

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit