net/netns, net/dns/resolver, etc: make netmon required in most places

The goal is to move more network state accessors to netmon.Monitor
where they can be cheaper/cached. But first (this change and others)
we need to make sure the one netmon.Monitor is plumbed everywhere.

Some notable bits:

* tsdial.NewDialer is added, taking a now-required netmon

* because a tsdial.Dialer always has a netmon, anything taking both
  a Dialer and a NetMon is now redundant; take only the Dialer and
  get the NetMon from that if/when needed.

* netmon.NewStatic is added, primarily for tests

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: I877f9cb87618c4eb037cee098241d18da9c01691
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/portmapper/igd_test.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/netaddr"
+	"tailscale.com/net/netmon"
 	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 )
@@ -259,12 +260,13 @@ func (d *TestIGD) handlePCPQuery(pkt []byte, src netip.AddrPort) {
 
 func newTestClient(t *testing.T, igd *TestIGD) *Client {
 	var c *Client
-	c = NewClient(t.Logf, nil, nil, new(controlknobs.Knobs), func() {
+	c = NewClient(t.Logf, netmon.NewStatic(), nil, new(controlknobs.Knobs), func() {
 		t.Logf("port map changed")
 		t.Logf("have mapping: %v", c.HaveMapping())
 	})
 	c.testPxPPort = igd.TestPxPPort()
 	c.testUPnPPort = igd.TestUPnPPort()
+	c.netMon = netmon.NewStatic()
 	c.SetGatewayLookupFunc(testIPAndGateway)
 	return c
 }

net/portmapper/portmapper.go

@@ -198,8 +198,7 @@ func (m *pmpMapping) Release(ctx context.Context) {
 
 // NewClient returns a new portmapping client.
 //
-// The netMon parameter is optional; if non-nil it's used to do faster interface
-// lookups.
+// The netMon parameter is required.
 //
 // The debug argument allows configuring the behaviour of the portmapper for
 // debugging; if nil, a sensible set of defaults will be used.
@@ -211,10 +210,13 @@ func (m *pmpMapping) Release(ctx context.Context) {
 // whenever the port mapping status has changed. If nil, it doesn't make a
 // callback.
 func NewClient(logf logger.Logf, netMon *netmon.Monitor, debug *DebugKnobs, controlKnobs *controlknobs.Knobs, onChange func()) *Client {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	ret := &Client{
 		logf:         logf,
 		netMon:       netMon,
-		ipAndGateway: interfaces.LikelyHomeRouterIP,
+		ipAndGateway: interfaces.LikelyHomeRouterIP, // TODO(bradfitz): move this to netMon
 		onChange:     onChange,
 		controlKnobs: controlKnobs,
 	}