Updates tailscale/tailscale#1634
Logs from some iOS users indicate that we're pointlessly performing captive portal detection on certain interfaces named ipsec*. These are tunnels with the cellular carrier that do not offer Internet access, and are only used to provide internet calling functionality (VoLTE / VoWiFi).
```
attempting to do captive portal detection on interface ipsec1
attempting to do captive portal detection on interface ipsec6
```
This PR excludes interfaces with the `ipsec` prefix from captive portal detection.
Signed-off-by: Andrea Gottardo <andrea@gottardo.me>
69be54c7b6