Update connstate tracking to support HSS

[drbild]

enftun/src/conn_state.c

Line 34 in db59a95

check_preferred_route(struct enftun_conn_state* conn_state)

For local TCP sockets, we:

• subscribe to netlink notifications for routing table changes
• when the routing table changes, we check if the preferred route has change by
  • opening and "connecting" a UDP socket
  • checking if the local address for that socket is the same as the active TCP tunnel
  • if addresses are different, our route has changed, so reconnect the tunnel (call to reconect_cb)

When connected via HSS, we want to detect if local TCP (wifi) becomes available:

• subscribe to netlink notifications for routing table changes
• when the routing table change, check if it is possible to connect locally (via wifi) by
  • opening and "connecting" a UDP socket
  • if that succeeds, reconnect the tunnel (call to reconnect_cb)

When connected va HSS, we want to detect if the host (HSS kernel driver) route has changed:

• TODO