Add option to preserve digests on copy

When enabled, if digests can't be preserved an error will be raised.

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

vendor/github.com/opencontainers/selinux/go-selinux/selinux_stub.go

@@ -17,10 +17,18 @@ func setFileLabel(fpath string, label string) error {
 	return nil
 }
 
+func lSetFileLabel(fpath string, label string) error {
+	return nil
+}
+
 func fileLabel(fpath string) (string, error) {
 	return "", nil
 }
 
+func lFileLabel(fpath string) (string, error) {
+	return "", nil
+}
+
 func setFSCreateLabel(label string) error {
 	return nil
 }