make do_attempt_reconnect work with route/ifconfig

make do_attempt_reconnect work with 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, we need to ensure that get_default_gw excludes
TUNDEV (since the goal is to prevent loopback), and only uses the first
match.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>