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>

cmd/derper/mesh.go

@@ -15,6 +15,7 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 )
@@ -36,7 +37,8 @@ func startMesh(s *derp.Server) error {
 
 func startMeshWithHost(s *derp.Server, host string) error {
 	logf := logger.WithPrefix(log.Printf, fmt.Sprintf("mesh(%q): ", host))
-	c, err := derphttp.NewClient(s.PrivateKey(), "https://"+host+"/derp", logf)
+	netMon := netmon.NewStatic() // good enough for cmd/derper; no need for netns fanciness
+	c, err := derphttp.NewClient(s.PrivateKey(), "https://"+host+"/derp", logf, netMon)
 	if err != nil {
 		return err
 	}