TUN-1615: revert miekg/dns to last known working revision

The last known good commit is 6da3249dfb57fbaa16efafcd8744cee8809d80cd before the first release tag.

vendor/golang.org/x/net/ipv4/sys_asmreqn.go

@@ -1,42 +0,0 @@
-// Copyright 2014 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 darwin freebsd linux
-
-package ipv4
-
-import (
-	"net"
-	"unsafe"
-
-	"golang.org/x/net/internal/socket"
-)
-
-func (so *sockOpt) getIPMreqn(c *socket.Conn) (*net.Interface, error) {
-	b := make([]byte, so.Len)
-	if _, err := so.Get(c, b); err != nil {
-		return nil, err
-	}
-	mreqn := (*ipMreqn)(unsafe.Pointer(&b[0]))
-	if mreqn.Ifindex == 0 {
-		return nil, nil
-	}
-	ifi, err := net.InterfaceByIndex(int(mreqn.Ifindex))
-	if err != nil {
-		return nil, err
-	}
-	return ifi, nil
-}
-
-func (so *sockOpt) setIPMreqn(c *socket.Conn, ifi *net.Interface, grp net.IP) error {
-	var mreqn ipMreqn
-	if ifi != nil {
-		mreqn.Ifindex = int32(ifi.Index)
-	}
-	if grp != nil {
-		mreqn.Multiaddr = [4]byte{grp[0], grp[1], grp[2], grp[3]}
-	}
-	b := (*[sizeofIPMreqn]byte)(unsafe.Pointer(&mreqn))[:sizeofIPMreqn]
-	return so.Set(c, b)
-}