Fill out the attempt-reconnect handler

• iproute2: Find all routes that match the VPN gateway (excluding those matching dev $TUNDEV, since the goal is to prevent loopback), and simply try adding them one-by-one.
• route/ifconfig: Unlike with iproute2, there is no way to determine which interface(s) have routes that actually match the VPN gateway, so we simply try finding a default-route gateway, as upon initial connection. For this to work properly on reconnection, a couple bugs in get_default_gw need to be fixed:
  1. Exclude $TUNDEV (since the goal is to prevent loopback)
  2. Only return the first default route found

See:

• openconnect#17 (closed) (motivation for existence of reason=attempt-reconnect)
• 1000e0f6 (addition of reason=attempt-reconnect stub here)
• https://github.com/dlenski/vpn-slice/pull/14#issuecomment-489293114 (discussion of some of the complexities of determining the correct route to the VPN gateway after a change in the underlying network devices)