net/tsdial: give netstack a Dialer, start refactoring name resolution

This starts to refactor tsdial.Dialer's name resolution to have
different stages: in-memory MagicDNS vs system resolution. A future
change will plug in ExitDNS resolution.

This also plumbs a Dialer into netstack and unexports the dnsMap
internals.

And it removes some of the async AddNetworkMapCallback usage and
replaces it with synchronous updates of the Dialer's netmap
from LocalBackend, since the LocalBackend has the Dialer too.

Updates #3475

Change-Id: Idcb7b1169878c74f0522f5151031ccbc49fe4cb4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsnet/tsnet.go

@@ -132,7 +132,7 @@ func (s *Server) start() error {
 		return fmt.Errorf("%T is not a wgengine.InternalsGetter", eng)
 	}
 
-	ns, err := netstack.Create(logf, tunDev, eng, magicConn)
+	ns, err := netstack.Create(logf, tunDev, eng, magicConn, dialer)
 	if err != nil {
 		return fmt.Errorf("netstack.Create: %w", err)
 	}