FileSystem::mkdir() should handle open_basedir correctly

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

The issue summary should be updated to clearly show what the proposed solution is, remaining tasks, etc

-        usleep((int) (($time_to_start - microtime(TRUE)) * 1000000));
+        usleep((int) (($time_to_start - microtime(TRUE)) * 5000000));

Not sure if this is in scope?

But as a bug this will need a test case showing the issue.

Thanks for the patch, without it, it is impossible to handle securing private files outside of the webroot with the open_basedir directive. After applying it, wet can set private and tmp using relative directories, such as:

```
$settings['file_private_path'] = '../../private';
$settings['file_temp_path'] = '../../tmp';
```

After some time getting to the bottom of #3411785, I would hope that we could get this sorted out. @smustgrave: is it possible to set up open_basedir in the testing environment? If not, I'm not clear how a test could be created for this issue.

I use shared hosting for a site, but I am able to use absolute paths, even when I set the file private path. Maybe the issue summary should explain when there are issues with using absolute paths.

#21 saved my day!

Patch #54 generate a segmentation fault if you try to create an folder with an empty name (infinite recursion).

drush ev '$f = Drupal::service("file_system");$dir="";var_dump($f->mkdir($dir, 1));'
Segmentation fault