unix: add support for OpenBSD pledge

Pledge, the privilege-restricting syscall and mitigation mechanism,
was missing from syscall_openbsd.go. As of the latest release, it
is officially supported in "stable".

More information about the call itself, and hence its importance,
can be found at:

http://www.openbsd.org/papers/hackfest2015-pledge/mgp00001.html

Change-Id: I2fdac1968664668e7bea1175677efe6433e0125e
Reviewed-on: https://go-review.googlesource.com/21815
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/openbsd_pledge.go

@@ -0,0 +1,38 @@
+// Copyright 2016 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 openbsd
+// +build 386 amd64 arm
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+const (
+	SYS_PLEDGE = 108
+)
+
+// Pledge implements the pledge syscall. For more information see pledge(2).
+func Pledge(promises string, paths []string) error {
+	promisesPtr, err := syscall.BytePtrFromString(promises)
+	if err != nil {
+		return err
+	}
+	promisesUnsafe, pathsUnsafe := unsafe.Pointer(promisesPtr), unsafe.Pointer(nil)
+	if paths != nil {
+		var pathsPtr []*byte
+		if pathsPtr, err = syscall.SlicePtrFromStrings(paths); err != nil {
+			return err
+		}
+		pathsUnsafe = unsafe.Pointer(&pathsPtr[0])
+	}
+	_, _, e := syscall.Syscall(SYS_PLEDGE, uintptr(promisesUnsafe), uintptr(pathsUnsafe), 0)
+	if e != 0 {
+		return e
+	}
+	return nil
+}