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/dnscache/dnscache.go

@@ -19,7 +19,6 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
-	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/singleflight"
@@ -90,11 +89,6 @@ type Resolver struct {
 	// be added to all log messages printed with this logger.
 	Logf logger.Logf
 
-	// NetMon optionally provides a netmon.Monitor to use to get the current
-	// (cached) network interface.
-	// If nil, the interface will be looked up dynamically.
-	NetMon *netmon.Monitor
-
 	sf singleflight.Group[string, ipRes]
 
 	mu      sync.Mutex