Re-vendor, primarily for https://github.com/containers/storage/pull/11

containers/storage got new dependencies, so we will need to re-vendor
eventually anyway, and having this separate from other major work is
cleaner.

But the primary goal of this commit is to see whether it makes skopeo
buildable on OS X.

vendor/golang.org/x/sys/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
+}