net/dns: rename Set to SetDNS in OSConfigurator.

wgengine/router.CallbackRouter needs to support both the Router
and OSConfigurator interfaces, so the setters can't both be called
Set.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/manager.go

@@ -86,7 +86,7 @@ func (m *Manager) Set(cfg Config) error {
 	if err := m.resolver.SetConfig(resolverCfg); err != nil {
 		return err
 	}
-	if err := m.os.Set(osCfg); err != nil {
+	if err := m.os.SetDNS(osCfg); err != nil {
 		return err
 	}