hostnet: create a `for_us` binding in the TCP path too
hostnet: create a `for_us` binding in the TCP path too

To make the code easier to read the UDP path declares a boolean
`for_us` which is true if the traffic is directly sent to us (rather
than through us as a gateway). This patch replicates this behaviour
in the TCP path.

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

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