feature/portmapper: make the portmapper & its debugging tools modular

Starting at a minimal binary and adding one feature back...
    tailscaled tailscale combined (linux/amd64)
     30073135  17451704  31543692 omitting everything
    +  480302 +   10258 +  493896 .. add debugportmapper
    +  475317 +  151943 +  467660 .. add portmapper
    +  500086 +  162873 +  510511 .. add portmapper+debugportmapper

Fixes #17148

Change-Id: I90bd0e9d1bd8cbe64fa2e885e9afef8fb5ee74b1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/portmapper/igd_test.go

@@ -14,7 +14,6 @@ import (
 	"sync/atomic"
 	"testing"
 
-	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/netmon"
 	"tailscale.com/syncs"
@@ -273,10 +272,9 @@ func newTestClient(t *testing.T, igd *TestIGD, bus *eventbus.Bus) *Client {
 	}
 	var c *Client
 	c = NewClient(Config{
-		Logf:         tstest.WhileTestRunningLogger(t),
-		NetMon:       netmon.NewStatic(),
-		ControlKnobs: new(controlknobs.Knobs),
-		EventBus:     bus,
+		Logf:     tstest.WhileTestRunningLogger(t),
+		NetMon:   netmon.NewStatic(),
+		EventBus: bus,
 		OnChange: func() { // TODO(creachadair): Remove.
 			t.Logf("port map changed")
 			t.Logf("have mapping: %v", c.HaveMapping())