When the desired netfilter mode was unset, we would always try
to use the `iptables` binary. In such cases if iptables was not found,
tailscaled would just crash as seen in #13440. To work around this, in those
cases check if the `iptables` binary even exists and if it doesn't fall back
to the nftables implementation.
Verified that it works on stock Ubuntu 24.04.
Updates #5621
Updates #8555
Updates #8762Fixes#13440
Signed-off-by: Maisem Ali <maisem@tailscale.com>
Just a refactor to consolidate the firewall detection logic in a single
package so that it can be reused in a later commit by containerboot.
Updates #9310
Signed-off-by: Maisem Ali <maisem@tailscale.com>
