unix: don't use 32-bit aligned access for cmsgAlignOf on dragonfly after ABI change

Use 32-bit alignment for versions before the September 2019 ABI changes
http://lists.dragonflybsd.org/pipermail/users/2019-September/358280.html

Follows CL 201977 which did the same for package syscall.

Updates golang/go#34958

Change-Id: I0e13fccf6563e4d34dd4aa7410be044881f220aa
Reviewed-on: https://go-review.googlesource.com/c/sys/+/202179
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/sockcmsg_dragonfly.go

@@ -0,0 +1,16 @@
+// Copyright 2019 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.
+
+package unix
+
+// Round the length of a raw sockaddr up to align it properly.
+func cmsgAlignOf(salen int) int {
+	salign := SizeofPtr
+	if SizeofPtr == 8 && !supportsABI(_dragonflyABIChangeVersion) {
+		// 64-bit Dragonfly before the September 2019 ABI changes still requires
+		// 32-bit aligned access to network subsystem.
+		salign = 4
+	}
+	return (salen + salign - 1) & ^(salign - 1)
+}