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>

control/controlhttp/client.go

@@ -393,7 +393,6 @@ func (a *Dialer) resolver() *dnscache.Resolver {
 		LookupIPFallback: dnsfallback.MakeLookupFunc(a.logf, a.NetMon),
 		UseLastGood:      true,
 		Logf:             a.Logf, // not a.logf method; we want to propagate nil-ness
-		NetMon:           a.NetMon,
 	}
 }
 
@@ -412,7 +411,6 @@ func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, addr netip.Addr,
 			SingleHostStaticResult: []netip.Addr{addr},
 			SingleHost:             u.Hostname(),
 			Logf:                   a.Logf, // not a.logf method; we want to propagate nil-ness
-			NetMon:                 a.NetMon,
 		}
 	} else {
 		dns = a.resolver()