Refactor TUN setup (isolated config)

src/tuntap/options.go

@@ -0,0 +1,20 @@
+package tuntap
+
+func (m *TunAdapter) _applyOption(opt SetupOption) {
+	switch v := opt.(type) {
+	case InterfaceName:
+		m.config.name = v
+	case InterfaceMTU:
+		m.config.mtu = v
+	}
+}
+
+type SetupOption interface {
+	isSetupOption()
+}
+
+type InterfaceName string
+type InterfaceMTU uint64
+
+func (a InterfaceName) isSetupOption() {}
+func (a InterfaceMTU) isSetupOption()  {}