When the network link changes, existing UDP sockets fail immediately
and permanently on macOS.
The forwarder set up a single UDP conn and never changed it.
As a result, any time there was a network link change,
all forwarded DNS queries failed.
To fix this, create a new connection when send requests
fail because of network unreachability.
This change is darwin-only, although extended it to other platforms
should be straightforward.
Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
