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/controlclient/noise_test.go

@@ -16,6 +16,7 @@ import (
 
 	"golang.org/x/net/http2"
 	"tailscale.com/control/controlhttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -73,7 +74,7 @@ func (tt noiseClientTest) run(t *testing.T) {
 	})
 	defer hs.Close()
 
-	dialer := new(tsdial.Dialer)
+	dialer := tsdial.NewDialer(netmon.NewStatic())
 	nc, err := NewNoiseClient(NoiseOpts{
 		PrivKey:      clientPrivate,
 		ServerPubKey: serverPrivate.Public(),