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>

feature/portmapper/portmapper.go

@@ -0,0 +1,38 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package portmapper registers support for NAT-PMP, PCP, and UPnP port
+// mapping protocols to help get direction connections through NATs.
+package portmapper
+
+import (
+	"tailscale.com/net/netmon"
+	"tailscale.com/net/portmapper"
+	"tailscale.com/net/portmapper/portmappertype"
+	"tailscale.com/types/logger"
+	"tailscale.com/util/eventbus"
+)
+
+func init() {
+	portmappertype.HookNewPortMapper.Set(newPortMapper)
+}
+
+func newPortMapper(
+	logf logger.Logf,
+	bus *eventbus.Bus,
+	netMon *netmon.Monitor,
+	disableUPnPOrNil func() bool,
+	onlyTCP443OrNil func() bool) portmappertype.Client {
+
+	pm := portmapper.NewClient(portmapper.Config{
+		EventBus: bus,
+		Logf:     logf,
+		NetMon:   netMon,
+		DebugKnobs: &portmapper.DebugKnobs{
+			DisableAll:      onlyTCP443OrNil,
+			DisableUPnPFunc: disableUPnPOrNil,
+		},
+	})
+	pm.SetGatewayLookupFunc(netMon.GatewayAndSelfIP)
+	return pm
+}