net/dns, feature/featuretags: make NetworkManager, systemd-resolved, and DBus modular

Saves 360 KB (19951800 => 19591352 on linux/amd64 --extra-small --box binary)

Updates #12614
Updates #17206

Change-Id: Iafd5b2536dd735111b447546cba335a7a64379ed
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolved.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build linux && !android
+//go:build linux && !android && !ts_omit_resolved
 
 package dns
 
@@ -70,7 +70,11 @@ type resolvedManager struct {
 	configCR chan changeRequest // tracks OSConfigs changes and error responses
 }
 
-func newResolvedManager(logf logger.Logf, health *health.Tracker, interfaceName string) (*resolvedManager, error) {
+func init() {
+	optNewResolvedManager.Set(newResolvedManager)
+}
+
+func newResolvedManager(logf logger.Logf, health *health.Tracker, interfaceName string) (OSConfigurator, error) {
 	iface, err := net.InterfaceByName(interfaceName)
 	if err != nil {
 		return nil, err