Update default interface MTU to use per-platform instead of global value

src/yggdrasil/tun_linux.go

@@ -12,7 +12,7 @@ import water "github.com/neilalexander/water"
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     65535,
-		defaultIfMTU:     DEFAULT_MTU,
+		defaultIfMTU:     65535,
 		defaultIfName:    "auto",
 		defaultIfTAPMode: false,
 	}