cmd/natc: add a flag to use specific DNS servers

If natc is running on a host with tailscale using `--accept-dns=true`
then a DNS loop can occur. Provide a flag for some specific DNS
upstreams for natc to use instead, to overcome such situations.

Updates #14667

Signed-off-by: James Tucker <james@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f