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>

control/controlhttp/http_test.go

@@ -22,6 +22,7 @@ import (
 
 	"tailscale.com/control/controlbase"
 	"tailscale.com/net/dnscache"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/socks5"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
@@ -199,14 +200,17 @@ func testControlHTTP(t *testing.T, param httpTestParam) {
 		defer cancel()
 	}
 
+	netMon := netmon.NewStatic()
+	dialer := tsdial.NewDialer(netMon)
 	a := &Dialer{
 		Hostname:             "localhost",
 		HTTPPort:             strconv.Itoa(httpLn.Addr().(*net.TCPAddr).Port),
 		HTTPSPort:            strconv.Itoa(httpsLn.Addr().(*net.TCPAddr).Port),
 		MachineKey:           client,
 		ControlKey:           server.Public(),
+		NetMon:               netMon,
 		ProtocolVersion:      testProtocolVersion,
-		Dialer:               new(tsdial.Dialer).SystemDial,
+		Dialer:               dialer.SystemDial,
 		Logf:                 t.Logf,
 		omitCertErrorLogging: true,
 		testFallbackDelay:    fallbackDelay,
@@ -643,7 +647,7 @@ func TestDialPlan(t *testing.T) {
 
 			dialer := closeTrackDialer{
 				t:     t,
-				inner: new(tsdial.Dialer).SystemDial,
+				inner: tsdial.NewDialer(netmon.NewStatic()).SystemDial,
 				conns: make(map[*closeTrackConn]bool),
 			}
 			defer dialer.Done()