Vendor in latest containers storage

We want to get support into skopeo for handling
override_kernel_checks so that we can use overlay
backend on RHEL.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

vendor/github.com/containers/storage/pkg/system/exitcode.go

@@ -0,0 +1,33 @@
+package system
+
+import (
+	"fmt"
+	"os/exec"
+	"syscall"
+)
+
+// GetExitCode returns the ExitStatus of the specified error if its type is
+// exec.ExitError, returns 0 and an error otherwise.
+func GetExitCode(err error) (int, error) {
+	exitCode := 0
+	if exiterr, ok := err.(*exec.ExitError); ok {
+		if procExit, ok := exiterr.Sys().(syscall.WaitStatus); ok {
+			return procExit.ExitStatus(), nil
+		}
+	}
+	return exitCode, fmt.Errorf("failed to get exit code")
+}
+
+// ProcessExitCode process the specified error and returns the exit status code
+// if the error was of type exec.ExitError, returns nothing otherwise.
+func ProcessExitCode(err error) (exitCode int) {
+	if err != nil {
+		var exiterr error
+		if exitCode, exiterr = GetExitCode(err); exiterr != nil {
+			// TODO: Fix this so we check the error's text.
+			// we've failed to retrieve exit code, so we set it to 127
+			exitCode = 127
+		}
+	}
+	return
+}