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>

net/netcheck/standalone.go

@@ -24,12 +24,15 @@ import (
 // to bind, errors will be returned, if one or both protocols can bind no error
 // is returned.
 func (c *Client) Standalone(ctx context.Context, bindAddr string) error {
+	if c.NetMon == nil {
+		panic("netcheck.Client.NetMon must be set")
+	}
 	if bindAddr == "" {
 		bindAddr = ":0"
 	}
 	var errs []error
 
-	u4, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, nil)).ListenPacket(ctx, "udp4", bindAddr)
+	u4, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, c.NetMon)).ListenPacket(ctx, "udp4", bindAddr)
 	if err != nil {
 		c.logf("udp4: %v", err)
 		errs = append(errs, err)
@@ -37,7 +40,7 @@ func (c *Client) Standalone(ctx context.Context, bindAddr string) error {
 		go readPackets(ctx, c.logf, u4, c.ReceiveSTUNPacket)
 	}
 
-	u6, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, nil)).ListenPacket(ctx, "udp6", bindAddr)
+	u6, err := nettype.MakePacketListenerWithNetIP(netns.Listener(c.logf, c.NetMon)).ListenPacket(ctx, "udp6", bindAddr)
 	if err != nil {
 		c.logf("udp6: %v", err)
 		errs = append(errs, err)