route: use libc calls on Darwin

Starting with 1.12, we must use syscall versions
of sysctl instead of the raw syscall.

An identical CL will go into the vendored copy of this package in the stdlib.

Change-Id: I6286ab3e49f82512491afb5bcf349e89ab5645ab
Reviewed-on: https://go-review.googlesource.com/c/148597
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

route/empty.s

+// Copyright 2018 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,go1.12
+
+// This exists solely so we can linkname in symbols from syscall.

route/syscall.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 darwin dragonfly freebsd netbsd openbsd
+// +build dragonfly freebsd netbsd openbsd
 
 package route
 

route/syscall_go1_11_darwin.go

+// Copyright 2018 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 !go1.12
+
+package route
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var zero uintptr
+
+func sysctl(mib []int32, old *byte, oldlen *uintptr, new *byte, newlen uintptr) error {
+	var p unsafe.Pointer
+	if len(mib) > 0 {
+		p = unsafe.Pointer(&mib[0])
+	} else {
+		p = unsafe.Pointer(&zero)
+	}
+	_, _, errno := syscall.Syscall6(syscall.SYS___SYSCTL, uintptr(p), uintptr(len(mib)), uintptr(unsafe.Pointer(old)), uintptr(unsafe.Pointer(oldlen)), uintptr(unsafe.Pointer(new)), newlen)
+	if errno != 0 {
+		return error(errno)
+	}
+	return nil
+}

route/syscall_go1_12_darwin.go

+// Copyright 2018 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 go1.12
+
+package route
+
+import _ "unsafe" // for linkname
+
+//go:linkname sysctl syscall.sysctl
+func sysctl(mib []int32, old *byte, oldlen *uintptr, new *byte, newlen uintptr) error