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/github.com/containers/storage/drivers/overlay/mount.go

@@ -0,0 +1,88 @@
+// +build linux
+
+package overlay
+
+import (
+	"bytes"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"syscall"
+
+	"github.com/containers/storage/pkg/reexec"
+)
+
+func init() {
+	reexec.Register("storage-mountfrom", mountFromMain)
+}
+
+func fatal(err error) {
+	fmt.Fprint(os.Stderr, err)
+	os.Exit(1)
+}
+
+type mountOptions struct {
+	Device string
+	Target string
+	Type   string
+	Label  string
+	Flag   uint32
+}
+
+func mountFrom(dir, device, target, mType, label string) error {
+	options := &mountOptions{
+		Device: device,
+		Target: target,
+		Type:   mType,
+		Flag:   0,
+		Label:  label,
+	}
+
+	cmd := reexec.Command("storage-mountfrom", dir)
+	w, err := cmd.StdinPipe()
+	if err != nil {
+		return fmt.Errorf("mountfrom error on pipe creation: %v", err)
+	}
+
+	output := bytes.NewBuffer(nil)
+	cmd.Stdout = output
+	cmd.Stderr = output
+
+	if err := cmd.Start(); err != nil {
+		return fmt.Errorf("mountfrom error on re-exec cmd: %v", err)
+	}
+	//write the options to the pipe for the untar exec to read
+	if err := json.NewEncoder(w).Encode(options); err != nil {
+		return fmt.Errorf("mountfrom json encode to pipe failed: %v", err)
+	}
+	w.Close()
+
+	if err := cmd.Wait(); err != nil {
+		return fmt.Errorf("mountfrom re-exec error: %v: output: %s", err, output)
+	}
+	return nil
+}
+
+// mountfromMain is the entry-point for storage-mountfrom on re-exec.
+func mountFromMain() {
+	runtime.LockOSThread()
+	flag.Parse()
+
+	var options *mountOptions
+
+	if err := json.NewDecoder(os.Stdin).Decode(&options); err != nil {
+		fatal(err)
+	}
+
+	if err := os.Chdir(flag.Arg(0)); err != nil {
+		fatal(err)
+	}
+
+	if err := syscall.Mount(options.Device, options.Target, options.Type, uintptr(options.Flag), options.Label); err != nil {
+		fatal(err)
+	}
+
+	os.Exit(0)
+}