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>

ipn/ipnlocal/local_test.go

@@ -2273,7 +2273,6 @@ func TestOnTailnetDefaultAutoUpdate(t *testing.T) {
 		t.Skip("test broken on macOS; see https://github.com/tailscale/tailscale/issues/11894")
 	}
 	tests := []struct {
-		desc           string
 		before, after  opt.Bool
 		tailnetDefault bool
 	}{
@@ -2309,7 +2308,7 @@ func TestOnTailnetDefaultAutoUpdate(t *testing.T) {
 		},
 	}
 	for _, tt := range tests {
-		t.Run(fmt.Sprintf("before=%s after=%s", tt.before, tt.after), func(t *testing.T) {
+		t.Run(fmt.Sprintf("before=%s,after=%s", tt.before, tt.after), func(t *testing.T) {
 			b := newTestBackend(t)
 			p := ipn.NewPrefs()
 			p.AutoUpdate.Apply = tt.before