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>

prober/derp_test.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 )
@@ -140,7 +141,7 @@ func TestRunDerpProbeNodePair(t *testing.T) {
 		}
 	}()
 	newClient := func() *derphttp.Client {
-		c, err := derphttp.NewClient(key.NewNode(), serverURL, t.Logf)
+		c, err := derphttp.NewClient(key.NewNode(), serverURL, t.Logf, netmon.NewStatic())
 		if err != nil {
 			t.Fatalf("NewClient: %v", err)
 		}