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/syscall_dragonfly.go

@@ -12,9 +12,25 @@
 
 package unix
 
-import "unsafe"
+import (
+	"sync"
+	"unsafe"
+)
 
-//sys	sysctl(mib []_C_int, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) = SYS___SYSCTL
+// See version list in https://github.com/DragonFlyBSD/DragonFlyBSD/blob/master/sys/sys/param.h
+var (
+	osreldateOnce sync.Once
+	osreldate     uint32
+)
+
+// First __DragonFly_version after September 2019 ABI changes
+// http://lists.dragonflybsd.org/pipermail/users/2019-September/358280.html
+const _dragonflyABIChangeVersion = 500705
+
+func supportsABI(ver uint32) bool {
+	osreldateOnce.Do(func() { osreldate, _ = SysctlUint32("kern.osreldate") })
+	return osreldate >= ver
+}
 
 // SockaddrDatalink implements the Sockaddr interface for AF_LINK type sockets.
 type SockaddrDatalink struct {