Replace deprecated uses

So my review,

A) The idea behind the issue is sound.

B) Al changes are of the form

- $realpath = realpath(dirname($path)) . '/' . drupal_basename($path);
+ $realpath = realpath(dirname($path)) . '/' . \Drupal::service('file_system')->basename($path);

which is the standard conversion I expected to see.

B) Formally I can say there are no changes outside this type of conversion.

C) Thinking about the possibility of opening up new securitty holes? always a worry with this module.

I don;t think the possibility exists as drupal_basename is just a wrapper for the service->basename call
so the two represent identical code flow.

This has already been fixed.