net/dns: add GetBaseConfig to OSConfigurator interface.

Part of #953, required to make split DNS work on more basic
platforms.

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

net/dns/nm.go

@@ -202,6 +202,11 @@ func (m nmManager) SetDNS(config OSConfig) error {
 
 func (m nmManager) SupportsSplitDNS() bool { return false }
 
+func (m nmManager) GetBaseConfig() (OSConfig, error) {
+	// TODO
+	return OSConfig{}, nil
+}
+
 func (m nmManager) Close() error {
 	return m.SetDNS(OSConfig{})
 }