syscall: fix FD passing on FreeBSD and NetBSD

Fixes #3348.

R=devon.odell, minux.ma, bradfitz, mdempsky
CC=golang-dev
https://golang.org/cl/7406050

src/pkg/syscall/passfd_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux darwin
+// +build linux darwin freebsd netbsd
 
 package syscall_test
 

src/pkg/syscall/sockcmsg_unix.go

@@ -37,7 +37,7 @@ func CmsgSpace(datalen int) int {
 }
 
 func cmsgData(h *Cmsghdr) unsafe.Pointer {
-	return unsafe.Pointer(uintptr(unsafe.Pointer(h)) + SizeofCmsghdr)
+	return unsafe.Pointer(uintptr(unsafe.Pointer(h)) + uintptr(cmsgAlignOf(SizeofCmsghdr)))
 }
 
 // SocketControlMessage represents a socket control message.