util/linuxfw, wgengine/router: display iptables version

[rodrigc]

Log the version of the iptables binary
to help better debug the logic in chooseFirewallMode() on different platforms.

Updates #391
Updates #9247

[rodrigc]

To test this:

1. I created a Vagrantfile based off of: https://app.vagrantup.com/irisstream/boxes/fedora-coreos

Vagrant.configure("2") do |config|
  config.vm.box = "irisstream/fedora-coreos"
  config.vm.box_version = "37.20230218.3.0"
  config.vm.provider "virtualbox" do |v|
      v.gui = true
  end
  config.ssh.username = "core"
end

2. vagrant up && vagrant ssh

3. Install go 1.21 inside this VM

4. Built this with:

GOOS=linux GOARCH=amd64 ./tool/go install tailscale.com/cmd/tailscale tailscale.com/cmd/tailscaled

5. Ran:

export TS_DEBUG_FIREWALL_MODE=auto
./tailscaled

6. In the logs, I saw:

wgengine.NewUserspaceEngine(tun "tailscale0") ...
router: detect iptables CLI version: iptables v1.8.9 (nf_tables)
router: detect iptables CLI version: ip6tables v1.8.9 (nf_tables)
router: nftables rule count: 0, iptables rule count: 0
router: nftables is available
router: using nftables
router: v6nat availability: true