all: avoid repeated default interface lookups

On some platforms (notably macOS and iOS) we look up the default
interface to bind outgoing connections to. This is both duplicated
work and results in logspam when the default interface is not available
(i.e. when a phone has no connectivity, we log an error and thus cause
more things that we will try to upload and fail).

Fixed by passing around a netmon.Monitor to more places, so that we can
use its cached interface state.

Fixes #7850
Updates #7621

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/dns/manager.go

@@ -64,6 +64,7 @@ type Manager struct {
 }
 
 // NewManagers created a new manager from the given config.
+// The netMon parameter is optional; if non-nil it's used to do faster interface lookups.
 func NewManager(logf logger.Logf, oscfg OSConfigurator, netMon *netmon.Monitor, dialer *tsdial.Dialer, linkSel resolver.ForwardLinkSelector) *Manager {
 	if dialer == nil {
 		panic("nil Dialer")