all: dns refactor, add Proxied and PerDomain flags from control (#615)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/dns/manager_linux.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+func newManager(mconfig ManagerConfig) managerImpl {
+	switch {
+	// systemd-resolved should only activate per-domain.
+	case isResolvedActive() && mconfig.PerDomain:
+		if mconfig.Cleanup {
+			return newNoopManager(mconfig)
+		} else {
+			return newResolvedManager(mconfig)
+		}
+	case isNMActive():
+		if mconfig.Cleanup {
+			return newNoopManager(mconfig)
+		} else {
+			return newNMManager(mconfig)
+		}
+	case isResolvconfActive():
+		return newResolvconfManager(mconfig)
+	default:
+		return newDirectManager(mconfig)
+	}
+}