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/ipv6/sys_asmreq.go

@@ -1,24 +0,0 @@
-// Copyright 2013 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 dragonfly freebsd linux netbsd openbsd solaris windows
-
-package ipv6
-
-import (
-	"net"
-	"unsafe"
-
-	"golang.org/x/net/internal/socket"
-)
-
-func (so *sockOpt) setIPMreq(c *socket.Conn, ifi *net.Interface, grp net.IP) error {
-	var mreq ipv6Mreq
-	copy(mreq.Multiaddr[:], grp)
-	if ifi != nil {
-		mreq.setIfindex(ifi.Index)
-	}
-	b := (*[sizeofIPv6Mreq]byte)(unsafe.Pointer(&mreq))[:sizeofIPv6Mreq]
-	return so.Set(c, b)
-}