tailcfg: Add FirewallMode to NetInfo to record wether host using iptables or nftables

To record wether user is using iptables or nftables after we add support to nftables on linux, we
are adding a field FirewallMode to NetInfo in HostInfo to reflect what firewall mode the host is
running, and form metrics. The information is gained from a global constant in hostinfo.go. We
set it when selection heuristic made the decision, and magicsock reports this to control.

Updates: tailscale/corp#13943
Signed-off-by: KevinLiang10 <kevinliang@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -640,6 +640,7 @@ func (c *Conn) updateNetInfo(ctx context.Context) (*netcheck.Report, error) {
 	if !c.setNearestDERP(ni.PreferredDERP) {
 		ni.PreferredDERP = 0
 	}
+	ni.FirewallMode = hostinfo.FirewallMode()
 
 	c.callNetInfoCallback(ni)
 	return report, nil