wgengine/router/dns: move to net/dns.

Preparation for merging the APIs and whatnot.

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

net/dns/manager_freebsd.go

@@ -0,0 +1,14 @@
+// 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 {
+	case isResolvconfActive():
+		return newResolvconfManager(mconfig)
+	default:
+		return newDirectManager(mconfig)
+	}
+}