dhcp-dns query using nmcli (for NetworkManager-managed DNS)

My setup doesn't use dhcpcd nor systemd-resolved, but rather NetworkManager.

While I haven't had a change to test this with captive portals yet, I suspect this command should work:

```
dhcp-dns = "nmcli -t -f IP4.DNS device show | cut -d ':' -f2 | head -n1"
```

Not certain if it's necessary limiting to the first line.

Sharing in case it's useful for anyone else. Maybe worth adding to the README? (Happy to PR)