net: add stubs for NetBSD

This fixes the build of package net for GOOS=NetBSD.
Of course, a real implementation would be even better.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5693065

src/pkg/net/sockoptip_netbsd.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build netbsd
+
+package net
+
+func setIPv4MulticastInterface(fd *netFD, ifi *Interface) error {
+	panic("unimplemented")
+}
+
+func setIPv4MulticastLoopback(fd *netFD, v bool) error {
+	panic("unimplemented")
+}